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Deletes the now-superseded sidebar shell, MinimizedWindowsBar, and the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/components/desktop-shell/TaskLauncher.svelte

@@ -0,0 +1,62 @@
+<script lang="ts">
+  // "What should Nomos do?" — the desktop's centerpiece. Extracted from the
+  // old Overview page hero so both the desktop surface and the Tasks app
+  // window can mount it; startTask() (see $lib/stores/chat.ts) begins the
+  // stream immediately and hands back the session id once the backend
+  // assigns one, which is the earliest point a task window can be opened.
+  import { startTask } from '$lib/stores/chat'
+  import { openTaskWindow } from '$lib/stores/windows'
+  import { truncateMiddle } from '$lib/utils'
+  import { Textarea } from '$lib/components/ui/textarea'
+  import { Button } from '$lib/components/ui/button'
+  import ArrowUpIcon from '@lucide/svelte/icons/arrow-up'
+
+  let { compact = false, onStarted }: { compact?: boolean; onStarted?: () => void } = $props()
+
+  let input = $state('')
+
+  function submit() {
+    const text = input.trim()
+    if (!text) return
+    input = ''
+    startTask(text, (sessionId) => openTaskWindow(sessionId, truncateMiddle(text, 60)))
+    onStarted?.()
+  }
+
+  function handleKeydown(e: KeyboardEvent) {
+    if (e.key === 'Enter' && !e.shiftKey) {
+      e.preventDefault()
+      submit()
+    }
+  }
+</script>
+
+<div class="w-full max-w-2xl text-center">
+  {#if !compact}
+    <h1 class="mb-1 text-2xl font-semibold tracking-tight">What should Nomos do?</h1>
+    <p class="mb-4 text-sm text-muted-foreground">
+      Describe a goal — Nomos will plan it, execute it, and report the outcome.
+    </p>
+  {/if}
+  <form
+    class="relative rounded-2xl border bg-card/70 shadow-lg backdrop-blur focus-within:border-primary/60"
+    onsubmit={(e) => {
+      e.preventDefault()
+      submit()
+    }}
+  >
+    <Textarea
+      bind:value={input}
+      onkeydown={handleKeydown}
+      placeholder="e.g. Roll the staging database back to last night's snapshot and verify the app is healthy…"
+      rows={compact ? 2 : 3}
+      class="max-h-52 min-h-24 resize-none border-0 bg-transparent px-4 py-3.5 text-base shadow-none focus-visible:ring-0"
+    />
+    <div class="flex items-center justify-between px-3 pb-3">
+      <span class="text-[11px] text-muted-foreground">Enter to start · Shift+Enter for newline</span>
+      <Button type="submit" size="icon" disabled={!input.trim()} aria-label="Start task">
+        <ArrowUpIcon />
+      </Button>
+    </div>
+  </form>
+</div>