feat(v0.6): auto-group Projects by namespace + Active-now band + cross-device prefs

The flat Projects grid was overwhelming at 50-100 repos. Explored the UX with
4 parallel agents (auto-grouping / manual folders / density-segmentation /
competitive research); all converged on namespace auto-grouping as the primary
fix — the dotted repo names (Billo.Platform.*, Billo.Infrastructure.*) already
encode the hierarchy, so it's a string-split, not a taxonomy the user builds.

Frontend:
- groupProjects(): depth-2 namespace grouping, >=2-member threshold (singletons
  fall to "Other"), degrade-to-flat when repos share no prefix, running projects
  duplicated into a pinned "Active now" section.
- displayLabel(): cards show the namespace tail (Payment, not
  Billo.Platform.Payment); full name in Active/Other.
- Collapsible sticky group headers with an "N active" badge so a collapsed
  section never hides running work; search force-expands matching groups.
- public/prefs.ts: server-first load with a localStorage offline mirror, migrates
  the legacy per-device proj-favs.

Backend (server-side prefs so favourites + collapse-state follow the user across
devices — localStorage was per-device, which broke the multi-device premise):
- src/http/prefs-store.ts: ~/.web-terminal-prefs.json store (mirrors push-store),
  every field bounded + sanitized.
- GET /prefs (read-only) + PUT /prefs (Origin-guarded); UiPrefs type +
  PREFS_STORE_PATH config.

Tests: +12 prefs-store, +21 groupProjects/displayLabel. Full suite 1227 passed.
Verified live: grouping renders, collapse persists to the server and survives a
full page reload (cross-device sync proven end-to-end).

src/server.ts

@@ -44,6 +44,7 @@ import { createWorktree } from './http/worktrees.js'
 import { createSessionManager } from './session/manager.js'
 import { detachWs, writeInput, setClientDims } from './session/session.js'
 import { loadSubscriptionStore } from './push/subscription-store.js'
+import { loadPrefsStore } from './http/prefs-store.js'
 import { createPushService } from './push/push-service.js'
 import type {
   Config,
@@ -190,6 +191,9 @@ export function startServer(cfg: Config): { close(): Promise<void> } {
   const pushService = createPushService(cfg, subStore)
   const manager = createSessionManager(cfg, pushService)
 
+  // v0.6 Projects: cross-device UI prefs (favourites + group collapse-state).
+  const prefsStore = loadPrefsStore(cfg.prefsStorePath)
+
   // Per-IP rate limiters for the new state-changing routes (SEC-H9).
   const decisionLimiter = createRateLimiter(DECISION_RATE_MAX, RATE_LIMIT_WINDOW_MS)
   const subscribeLimiter = createRateLimiter(SUBSCRIBE_RATE_MAX, RATE_LIMIT_WINDOW_MS)
@@ -264,6 +268,25 @@ export function startServer(cfg: Config): { close(): Promise<void> } {
     }
   })
 
+  // Projects UI prefs (v0.6) — favourites + group collapse-state, cross-device.
+  // GET is read-only (no secrets, just paths/booleans) → no Origin guard, like /projects.
+  app.get('/prefs', (_req, res) => {
+    res.json(prefsStore.get())
+  })
+  // PUT replaces the whole prefs blob. State-changing → Origin guard (CSRF, like
+  // /open-in-editor). Body is sanitized inside prefsStore.set (never trust it).
+  app.put('/prefs', express.json({ limit: '64kb' }), async (req, res) => {
+    if (!requireAllowedOrigin(req, res)) return
+    try {
+      prefsStore.set(req.body)
+      await prefsStore.persist()
+      res.json(prefsStore.get())
+    } catch (err) {
+      console.error('[server] PUT /prefs failed:', err instanceof Error ? err.message : String(err))
+      res.status(500).json({ error: 'failed to save prefs' })
+    }
+  })
+
   // Project detail (v0.6) — branch/worktrees + running sessions for one repo.
   // Read-only (git branch/status/worktree-list); same threat model as /projects.
   // ?path must be an absolute existing directory (validated in buildProjectDetail).