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Deletes the now-superseded sidebar shell, MinimizedWindowsBar, and the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/DesktopIcon.svelte

@@ -0,0 +1,100 @@
+<script lang="ts">
+  // A single desktop icon: positioned from the grid store, draggable to any
+  // free cell, opens its app on a plain click. wmkit has nothing to do with
+  // icons — they're a flat non-overlapping grid, not floating/resizable
+  // windows, so this is a small self-contained pointer-drag implementation
+  // rather than pressing wmkit's window abstractions into a shape they don't
+  // fit. See $lib/stores/icons.ts for the grid model + persistence.
+  import { GRID, iconPixelPos, placeIcon, type IconPos } from '$lib/stores/icons'
+  import type { AppDef } from '$lib/apps'
+
+  let {
+    app,
+    pos,
+    badge = 0,
+    onOpen
+  }: { app: AppDef; pos: IconPos; badge?: number; onOpen: () => void } = $props()
+
+  const DRAG_THRESHOLD = 5
+
+  let dragging = $state(false)
+  let dragPos = $state<{ x: number; y: number } | null>(null)
+
+  function toCell(x: number, y: number): IconPos {
+    return {
+      col: Math.round((x - GRID.padding) / (GRID.cell + GRID.gap)),
+      row: Math.round((y - GRID.padding) / (GRID.cell + GRID.gap))
+    }
+  }
+
+  function onPointerDown(e: PointerEvent) {
+    if (e.button !== 0) return
+    const el = e.currentTarget as HTMLElement
+    const startX = e.clientX
+    const startY = e.clientY
+    const origin = iconPixelPos(pos)
+    let moved = false
+
+    el.setPointerCapture(e.pointerId)
+
+    function onMove(ev: PointerEvent) {
+      const dx = ev.clientX - startX
+      const dy = ev.clientY - startY
+      if (!moved && Math.hypot(dx, dy) > DRAG_THRESHOLD) {
+        moved = true
+        dragging = true
+      }
+      if (moved) {
+        dragPos = { x: origin.x + dx, y: origin.y + dy }
+      }
+    }
+
+    function onUp() {
+      el.removeEventListener('pointermove', onMove)
+      el.removeEventListener('pointerup', onUp)
+      if (moved && dragPos) {
+        const cell = toCell(dragPos.x, dragPos.y)
+        placeIcon(app.id, cell.col, cell.row)
+      } else {
+        onOpen()
+      }
+      dragging = false
+      dragPos = null
+    }
+
+    el.addEventListener('pointermove', onMove)
+    el.addEventListener('pointerup', onUp)
+  }
+
+  function onKeydown(e: KeyboardEvent) {
+    if (e.key === 'Enter' || e.key === ' ') {
+      e.preventDefault()
+      onOpen()
+    }
+  }
+
+  const restPos = $derived(iconPixelPos(pos))
+  const left = $derived(dragging && dragPos ? dragPos.x : restPos.x)
+  const top = $derived(dragging && dragPos ? dragPos.y : restPos.y)
+</script>
+
+<button
+  type="button"
+  class="group absolute flex flex-col items-center gap-1 rounded-lg p-1.5 pointer-events-auto select-none focus-visible:outline-2 focus-visible:outline-ring {dragging
+    ? 'z-50 cursor-grabbing bg-accent/40'
+    : 'cursor-pointer hover:bg-accent/30'}"
+  style="left: {left}px; top: {top}px; width: {GRID.cell}px;"
+  onpointerdown={onPointerDown}
+  onkeydown={onKeydown}
+  title={app.title}
+>
+  <span class="relative flex size-10 items-center justify-center rounded-xl border bg-card/80 text-foreground shadow-sm backdrop-blur">
+    <app.icon class="size-5" />
+    {#if badge > 0}
+      <span class="absolute -right-1.5 -top-1.5 flex h-4 min-w-4 items-center justify-center rounded-full bg-destructive px-1 text-[10px] font-semibold text-destructive-foreground">
+        {badge > 99 ? '99+' : badge}
+      </span>
+    {/if}
+  </span>
+  <span class="max-w-full truncate text-[11px] text-foreground/90">{app.title}</span>
+</button>