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Deletes the now-superseded sidebar shell, MinimizedWindowsBar, and the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

@@ -0,0 +1,131 @@
+<script lang="ts">
+  // Bottom taskbar: a real flex row in the page layout (not an overlay), so
+  // windows can never be dragged/maximized underneath it — see Desktop.svelte
+  // for how the window layer's bounds are scoped to the surface above this.
+  // Shows a button for every open window (not just minimized ones — compare
+  // to the old MinimizedWindowsBar, which only ever showed minimized windows
+  // and gave no way to see/switch between windows that were merely
+  // unfocused), plus a system tray for theme/connection/version.
+  import { wm, wmState, toggleShowDesktop } from '$lib/stores/windows'
+  import { appById, appIdFromWindowId } from '$lib/apps'
+  import { summary } from '$lib/stores/context'
+  import { truncateMiddle } from '$lib/utils'
+  import { getTheme, toggleTheme, THEME_LABELS } from '$lib/stores/theme.svelte'
+  import { VERSION } from '$lib/version'
+  import MessageSquareIcon from '@lucide/svelte/icons/message-square'
+  import DatabaseIcon from '@lucide/svelte/icons/database'
+  import PaletteIcon from '@lucide/svelte/icons/palette'
+  import SettingsIcon from '@lucide/svelte/icons/settings'
+  import LayoutGridIcon from '@lucide/svelte/icons/layout-grid'
+  import XIcon from '@lucide/svelte/icons/x'
+
+  let { onOpenConnection }: { onOpenConnection: () => void } = $props()
+
+  const SESSION_PREFIX = 'session:'
+
+  const buttons = $derived(
+    [...$wmState.order]
+      .map((id) => $wmState.windows[id])
+      .filter((w): w is NonNullable<typeof w> => !!w)
+      .sort((a, b) => a.openedSeq - b.openedSeq)
+  )
+
+  function iconFor(id: string) {
+    const appId = appIdFromWindowId(id)
+    if (appId) return appById.get(appId)?.icon
+    if (id.startsWith(SESSION_PREFIX)) return MessageSquareIcon
+    return DatabaseIcon
+  }
+
+  function badgeFor(id: string): number {
+    const appId = appIdFromWindowId(id)
+    const app = appId ? appById.get(appId) : undefined
+    return app?.badge?.($summary) ?? 0
+  }
+
+  function toggle(id: string, win: (typeof buttons)[number]) {
+    if (win.stage === 'minimized') {
+      wm.restore(id)
+      wm.focus(id)
+    } else if ($wmState.focusedId === id) {
+      wm.minimize(id)
+    } else {
+      wm.focus(id)
+    }
+  }
+
+</script>
+
+<div class="flex h-11 shrink-0 items-center gap-1.5 border-t bg-muted/30 px-2">
+  <button
+    type="button"
+    class="flex shrink-0 items-center justify-center rounded-md p-1.5 text-muted-foreground hover:bg-muted hover:text-foreground"
+    onclick={toggleShowDesktop}
+    title="Show desktop"
+  >
+    <LayoutGridIcon class="size-4" />
+  </button>
+
+  <div class="h-6 w-px shrink-0 bg-border"></div>
+
+  <div class="flex min-w-0 flex-1 items-center gap-1 overflow-x-auto">
+    {#each buttons as win (win.id)}
+      {@const Icon = iconFor(win.id)}
+      {@const badge = badgeFor(win.id)}
+      <div class="group/tb relative flex shrink-0">
+        <button
+          type="button"
+          data-taskbar-btn={win.id}
+          class="flex h-8 max-w-56 items-center gap-1.5 rounded-md border px-2 font-mono text-xs transition-colors {$wmState.focusedId ===
+            win.id && win.stage !== 'minimized'
+            ? 'border-primary/50 bg-primary/10 text-foreground'
+            : 'border-transparent bg-card/60 text-muted-foreground hover:bg-muted'} {win.stage === 'minimized' ? 'opacity-60' : ''}"
+          onclick={() => toggle(win.id, win)}
+          title={win.title}
+        >
+          {#if Icon}<Icon class="size-3.5 shrink-0" />{/if}
+          <span class="min-w-0 truncate">{truncateMiddle(win.title, 26)}</span>
+          {#if badge > 0}
+            <span class="flex h-3.5 min-w-3.5 shrink-0 items-center justify-center rounded-full bg-destructive px-1 text-[9px] font-semibold text-destructive-foreground">
+              {badge > 99 ? '99+' : badge}
+            </span>
+          {/if}
+        </button>
+        <button
+          type="button"
+          class="absolute -top-1.5 -right-1.5 hidden size-4 items-center justify-center rounded-full bg-muted-foreground/80 text-background hover:bg-destructive group-hover/tb:flex"
+          onclick={(e) => {
+            e.stopPropagation()
+            wm.close(win.id)
+          }}
+          aria-label="Close {win.title}"
+        >
+          <XIcon class="size-2.5" />
+        </button>
+      </div>
+    {/each}
+  </div>
+
+  <div class="h-6 w-px shrink-0 bg-border"></div>
+
+  <div class="flex shrink-0 items-center gap-0.5">
+    <button
+      type="button"
+      class="flex items-center justify-center gap-1.5 rounded-md p-1.5 text-muted-foreground hover:bg-muted hover:text-foreground"
+      onclick={() => toggleTheme()}
+      title="Cycle theme"
+    >
+      <PaletteIcon class="size-4" />
+      <span class="hidden text-xs sm:inline">{THEME_LABELS[getTheme()]}</span>
+    </button>
+    <button
+      type="button"
+      class="flex items-center justify-center rounded-md p-1.5 text-muted-foreground hover:bg-muted hover:text-foreground"
+      onclick={onOpenConnection}
+      title="Server connection settings"
+    >
+      <SettingsIcon class="size-4" />
+    </button>
+    <span class="px-1.5 text-[11px] text-muted-foreground select-none">{VERSION}</span>
+  </div>
+</div>