feat(web): redesign UI as an OS-style desktop shell

Replace the sidebar + hash-routed page shell with a desktop metaphor:
draggable app icons, apps opening as floating wmkit windows, a centered
"What should Nomos do?" task launcher, and a bottom taskbar showing all
open windows plus a system tray.

- New app registry ($lib/apps.ts) — adding an app is one entry, nothing
  else to touch.
- New desktop shell components (Desktop, WindowLayer, DesktopIcon,
  Taskbar, TaskLauncher) under $lib/components/desktop-shell/.
- Icon positions are a persisted, collision-avoiding grid ($lib/stores/icons.ts).
- Window layout persists across reloads (wmkit persist), with
  drag-to-maximize, F6 window cycling, and now a right-click desktop menu
  (cascade/tile/show desktop/reset icons) plus Cmd/Ctrl+Z undo/redo for
  window moves, resizes, and closes.
- Taskbar buttons get a hover-close and self-correct their title once a
  new task's real goal is known.
- New task windows (desktop launcher and the Tasks app's "New task"
  button) open as a window, not a dialog, and hand off to the real
  session window once the backend assigns an id.
- Fixed a real gap along the way: GET /sessions/{id} couldn't tell
  "session deleted" from "session has no messages yet" (both returned
  200 with an empty list) — cmd/nomos/main.go now checks existence and
  404s, so a stale/persisted task window shows "Task not found" instead
  of a misleadingly empty, live-looking chat.
- Test coverage for the new pure logic (icon placement/collision
  avoidance, app registry id helpers) plus a vitest matchMedia polyfill
  needed to import anything touching the theme store.

Deletes the now-superseded sidebar shell, MinimizedWindowsBar, and the
standalone Chat/EntityDetail pages (folded into the window layer).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/lib/stores/chat.ts

@@ -1,5 +1,5 @@
 import { writable, get, type Writable } from 'svelte/store'
-import { streamChat, fetchSessions, fetchMessages, deleteSession as apiDeleteSession } from '$lib/api'
+import { streamChat, fetchSessions, fetchMessages, fetchMessagesOrNotFound, deleteSession as apiDeleteSession } from '$lib/api'
 import type { ChatEvent, Session, Message } from '$lib/api'
 import type { ToolCallResult } from '$lib/types'
 
@@ -499,6 +499,11 @@ export interface SessionChatState {
   streaming: Writable<boolean>
   connectionState: Writable<'connected' | 'disconnected' | 'reconnecting'>
   error: Writable<string | null>
+  // Set by loadSessionChat when the backend 404s the session outright
+  // (deleted, or an id that was never valid — a stale persisted window, a
+  // bad deep link). Distinct from a merely-empty transcript, which is the
+  // normal state for a session that exists but hasn't sent a message yet.
+  notFound: Writable<boolean>
 }
 
 const sessionChats = new Map<string, SessionChatState>()
@@ -509,7 +514,13 @@ const sessionPollers = new Map<string, ReturnType<typeof setInterval>>()
 export function chatFor(sessionId: string): SessionChatState {
   let c = sessionChats.get(sessionId)
   if (!c) {
-    c = { messages: writable([]), streaming: writable(false), connectionState: writable('connected'), error: writable(null) }
+    c = {
+      messages: writable([]),
+      streaming: writable(false),
+      connectionState: writable('connected'),
+      error: writable(null),
+      notFound: writable(false)
+    }
     sessionChats.set(sessionId, c)
   }
   return c
@@ -544,7 +555,11 @@ export function stopSessionPolling(sessionId: string) {
 export async function loadSessionChat(sessionId: string): Promise<void> {
   const chat = chatFor(sessionId)
   chat.streaming.set(false)
-  const msgs = await fetchMessages(sessionId)
+  const msgs = await fetchMessagesOrNotFound(sessionId)
+  if (msgs === null) {
+    chat.notFound.set(true)
+    return // nothing to poll — the session doesn't exist
+  }
   chat.messages.set(toChatMessages(msgs))
   startSessionPolling(sessionId)
 }
@@ -650,3 +665,117 @@ export function cancelSessionStream(sessionId: string) {
   activeControllers.delete(sessionId)
   chatFor(sessionId).streaming.set(false)
 }
+
+// ─── new-task launcher (desktop center input / Tasks app) ───────────────────
+//
+// Starting a brand-new task has no session id to hang a window off of until
+// the stream's own 'session' event assigns one (see the 'session' branch in
+// sendMessage above) — the desktop launcher needs to open that task's window
+// the moment an id exists, not before. startTask begins the stream
+// immediately, buffers any events that arrive before 'session' (defensive:
+// in practice 'session' always arrives first), then seeds that session's own
+// chatFor() bundle exactly like sendSessionMessage does and hands the id back
+// via onSession so the caller can open its window. From that point on the
+// window behaves exactly like any other task window.
+export function startTask(text: string, onSession: (sessionId: string) => void): void {
+  const userMsg: ChatMessage = { id: mid(), role: 'user', text, tools: [], pendingApprovals: [] }
+  const assistantMsg: ChatMessage = { id: mid(), role: 'assistant', text: '', tools: [], pendingApprovals: [] }
+  const activeTools: Map<string, ToolCallResult> = new Map()
+  let receivedDone = false
+  let sessionId: string | null = null
+  let chat: SessionChatState | null = null
+  const buffered: ChatEvent[] = []
+
+  function apply(ev: ChatEvent) {
+    const c = chat
+    if (!c || !sessionId) return
+    if (ev.type === 'tool_use') {
+      const tr: ToolCallResult = { type: 'tool_use', name: ev.data.name, id: ev.data.id, args: ev.data.args }
+      activeTools.set(ev.data.id, tr)
+      c.messages.update((ms) => {
+        const last = ms[ms.length - 1]
+        if (last && last.role === 'assistant') last.tools = [...last.tools, tr]
+        return [...ms]
+      })
+    } else if (ev.type === 'tool_result') {
+      const existing = activeTools.get(ev.data.id)
+      if (existing) {
+        const updated: ToolCallResult = { ...existing, type: 'tool_result', result: ev.data.result, error: ev.data.error }
+        activeTools.set(ev.data.id, updated)
+        c.messages.update((ms) => {
+          const last = ms[ms.length - 1]
+          if (last && last.role === 'assistant') {
+            last.tools = last.tools.map((t) => (t.id === ev.data.id ? updated : t))
+            last.pendingApprovals = extractApprovals(last.tools)
+          }
+          return [...ms]
+        })
+      }
+    } else if (ev.type === 'text_delta') {
+      c.messages.update((ms) => {
+        const last = ms[ms.length - 1]
+        if (last && last.role === 'assistant') last.text += ev.data
+        return [...ms]
+      })
+    } else if (ev.type === 'text') {
+      c.messages.update((ms) => {
+        const last = ms[ms.length - 1]
+        if (last && last.role === 'assistant') last.text = ev.data
+        return [...ms]
+      })
+    } else if (ev.type === 'done') {
+      receivedDone = true
+      c.connectionState.set('connected')
+      c.messages.update((ms) => {
+        const last = ms[ms.length - 1]
+        if (last && last.role === 'assistant') last.pendingApprovals = extractApprovals(last.tools)
+        return [...ms]
+      })
+      startSessionPolling(sessionId)
+    } else if (ev.type === 'error') {
+      c.error.set(ev.data)
+    }
+  }
+
+  const controller = streamChat(
+    text,
+    null,
+    (ev: ChatEvent) => {
+      if (ev.type === 'session') {
+        sessionId = ev.data
+        activeControllers.set(sessionId, controller)
+        chat = chatFor(sessionId)
+        chat.streaming.set(true)
+        chat.messages.update((ms) => [...ms, userMsg, assistantMsg])
+        onSession(sessionId)
+        for (const b of buffered.splice(0)) apply(b)
+        return
+      }
+      if (!chat) {
+        buffered.push(ev)
+        return
+      }
+      apply(ev)
+    },
+    (err: string) => {
+      if (!chat) return // never got a session id — nothing to show the error in
+      if (err === 'AbortError' || err.includes('aborted')) {
+        chat.streaming.set(false)
+        return
+      }
+      chat.error.set(err)
+      if (!receivedDone && sessionId) {
+        chat.connectionState.set('disconnected')
+        startSessionPolling(sessionId)
+        addChatError('Agent connection lost. The task is still running — retrying…', 'Dismiss')
+      } else {
+        chat.streaming.set(false)
+      }
+    },
+    () => {
+      if (chat) chat.streaming.set(false)
+      if (sessionId && activeControllers.get(sessionId) === controller) activeControllers.delete(sessionId)
+      loadSessions()
+    }
+  )
+}