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Deletes the now-superseded sidebar shell, MinimizedWindowsBar, and the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

@@ -0,0 +1,163 @@
+<script lang="ts">
+  // The desktop shell: full-viewport surface (background + icons + the
+  // centered task launcher + the floating window layer) with the taskbar
+  // docked below it as a real flex sibling, not an overlay — so a maximized
+  // or dragged window can never end up underneath the taskbar. This replaces
+  // the old sidebar + hash-routed page shell in App.svelte entirely; apps are
+  // desktop icons now (see $lib/apps.ts), not nav items.
+  import { APPS } from '$lib/apps'
+  import { iconPositions, resetIconLayout } from '$lib/stores/icons'
+  import { wm, openAppWindow, toggleShowDesktop } from '$lib/stores/windows'
+  import { summary } from '$lib/stores/context'
+  import GraphBackground from '../GraphBackground.svelte'
+  import DesktopIcon from './DesktopIcon.svelte'
+  import TaskLauncher from './TaskLauncher.svelte'
+  import WindowLayer from './WindowLayer.svelte'
+  import Taskbar from './Taskbar.svelte'
+  import LayersIcon from '@lucide/svelte/icons/layers'
+  import Rows3Icon from '@lucide/svelte/icons/rows-3'
+  import MonitorIcon from '@lucide/svelte/icons/monitor'
+  import RotateCcwIcon from '@lucide/svelte/icons/rotate-ccw'
+  import Undo2Icon from '@lucide/svelte/icons/undo-2'
+  import Redo2Icon from '@lucide/svelte/icons/redo-2'
+
+  let { onOpenConnection }: { onOpenConnection: () => void } = $props()
+
+  // Clicking the bare desktop (not an icon, not a window) blurs the focused
+  // window — the familiar "click empty desktop to deselect" affordance.
+  function onSurfaceClick(e: MouseEvent) {
+    if (e.currentTarget === e.target) wm.blur()
+  }
+
+  // Right-click menu, bare desktop only (same currentTarget===target gate as
+  // onSurfaceClick above — icons and windows sit on pointer-events-auto
+  // layers above the otherwise pointer-events-none surface, so a right-click
+  // that lands on either of them never reaches here). canUndo/canRedo are
+  // plain wmkit method calls (not stores), so they're snapshotted once at
+  // open time rather than read reactively in the template.
+  let menuPos = $state<{ x: number; y: number } | null>(null)
+  let menuCanUndo = $state(false)
+  let menuCanRedo = $state(false)
+
+  function onSurfaceContextMenu(e: MouseEvent) {
+    if (e.currentTarget !== e.target) return
+    e.preventDefault()
+    menuCanUndo = wm.canUndo()
+    menuCanRedo = wm.canRedo()
+    menuPos = { x: e.clientX, y: e.clientY }
+  }
+
+  function closeMenu() {
+    menuPos = null
+  }
+
+  function runMenuAction(fn: () => void) {
+    fn()
+    closeMenu()
+  }
+
+  // Cmd/Ctrl+Z / Shift+Z for window-arrangement undo/redo (move, resize,
+  // close, ...) — wmkit tracks this history but ships no default keybinding.
+  // Skipped entirely while an editable element has focus so it never
+  // fights the browser's own text-undo inside the task input or a form
+  // field.
+  function onWindowKeydown(e: KeyboardEvent) {
+    if (e.key === 'Escape' && menuPos) {
+      closeMenu()
+      return
+    }
+    const target = e.target as HTMLElement | null
+    const editable = !!target && (target.tagName === 'INPUT' || target.tagName === 'TEXTAREA' || target.isContentEditable)
+    if (editable) return
+    if (!(e.metaKey || e.ctrlKey) || e.key.toLowerCase() !== 'z') return
+    e.preventDefault()
+    if (e.shiftKey) wm.redo()
+    else wm.undo()
+  }
+</script>
+
+<svelte:window onkeydown={onWindowKeydown} onclick={closeMenu} />
+
+<div class="fixed inset-0 flex flex-col">
+  <div
+    class="relative min-h-0 flex-1 overflow-hidden"
+    role="presentation"
+    onclick={onSurfaceClick}
+    oncontextmenu={onSurfaceContextMenu}
+  >
+    <GraphBackground />
+
+    <div class="pointer-events-none absolute inset-0 z-0">
+      {#each APPS as app (app.id)}
+        {@const pos = $iconPositions[app.id] ?? { col: 0, row: 0 }}
+        {@const badge = app.badge?.($summary) ?? 0}
+        <DesktopIcon {app} {pos} {badge} onOpen={() => openAppWindow(app.id)} />
+      {/each}
+    </div>
+
+    <div class="pointer-events-none absolute inset-0 z-10 flex items-center justify-center p-6">
+      <div class="pointer-events-auto">
+        <TaskLauncher />
+      </div>
+    </div>
+
+    <WindowLayer />
+  </div>
+
+  <Taskbar {onOpenConnection} />
+</div>
+
+{#if menuPos}
+  <div
+    class="fixed z-50 min-w-48 rounded-md border bg-popover p-1 text-sm text-popover-foreground shadow-md ring-1 ring-foreground/10"
+    style="left: {menuPos.x}px; top: {menuPos.y}px"
+    role="menu"
+  >
+    <button
+      type="button"
+      class="flex w-full items-center gap-2 rounded-sm px-2 py-1.5 text-left hover:bg-accent hover:text-accent-foreground"
+      onclick={() => runMenuAction(() => wm.arrange('cascade'))}
+    >
+      <LayersIcon class="size-4" /> Cascade windows
+    </button>
+    <button
+      type="button"
+      class="flex w-full items-center gap-2 rounded-sm px-2 py-1.5 text-left hover:bg-accent hover:text-accent-foreground"
+      onclick={() => runMenuAction(() => wm.arrange('tile'))}
+    >
+      <Rows3Icon class="size-4" /> Tile windows
+    </button>
+    <button
+      type="button"
+      class="flex w-full items-center gap-2 rounded-sm px-2 py-1.5 text-left hover:bg-accent hover:text-accent-foreground"
+      onclick={() => runMenuAction(toggleShowDesktop)}
+    >
+      <MonitorIcon class="size-4" /> Show desktop
+    </button>
+    <div class="my-1 h-px bg-border"></div>
+    <button
+      type="button"
+      class="flex w-full items-center gap-2 rounded-sm px-2 py-1.5 text-left hover:bg-accent hover:text-accent-foreground"
+      onclick={() => runMenuAction(resetIconLayout)}
+    >
+      <RotateCcwIcon class="size-4" /> Reset icon layout
+    </button>
+    <div class="my-1 h-px bg-border"></div>
+    <button
+      type="button"
+      disabled={!menuCanUndo}
+      class="flex w-full items-center gap-2 rounded-sm px-2 py-1.5 text-left hover:bg-accent hover:text-accent-foreground disabled:pointer-events-none disabled:opacity-50"
+      onclick={() => runMenuAction(() => wm.undo())}
+    >
+      <Undo2Icon class="size-4" /> Undo
+    </button>
+    <button
+      type="button"
+      disabled={!menuCanRedo}
+      class="flex w-full items-center gap-2 rounded-sm px-2 py-1.5 text-left hover:bg-accent hover:text-accent-foreground disabled:pointer-events-none disabled:opacity-50"
+      onclick={() => runMenuAction(() => wm.redo())}
+    >
+      <Redo2Icon class="size-4" /> Redo
+    </button>
+  </div>
+{/if}