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/types.ts

@@ -47,6 +47,7 @@ export interface Config {
   readonly vapidSubject: string | undefined; // VAPID_SUBJECT (mailto:/url)
   readonly pushStorePath: string; // PUSH_STORE_PATH, default ~/.web-terminal-push-subs.json
   readonly pushMaxSubs: number; // PUSH_MAX_SUBS, default 50
+  readonly prefsStorePath: string; // PREFS_STORE_PATH, default ~/.web-terminal-prefs.json (Projects UI prefs)
   readonly notifyDone: boolean; // NOTIFY_DONE, default true
   readonly notifyDnd: boolean; // NOTIFY_DND, default false
   readonly decisionTokenTtlMs: number; // DECISION_TOKEN_TTL_MS, default = permTimeoutMs
@@ -483,6 +484,17 @@ export interface CreateWorktreeResult {
   error?: string;
 }
 
+/* ── v0.6 Projects UI preferences (server-persisted, cross-device) ── */
+
+/** Cross-device UI preferences for the Projects launcher (impl: src/http/prefs-store.ts).
+ *  Persisted host-side so favourites + group collapse-state follow the user from
+ *  laptop to phone (localStorage was per-device). Holds NO secrets — just project
+ *  paths and collapse booleans; served over GET /prefs, written via PUT /prefs. */
+export interface UiPrefs {
+  favourites: string[]; // favourited project paths (★), replaces per-device proj-favs
+  collapsed: Record<string, boolean>; // namespace group-key → collapsed? (true = hidden)
+}
+
 /* ── GET /config/ui (review #4) ── */
 
 /** Client-readable UI configuration (GET /config/ui). `allowAutoMode` mirrors