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/config.ts

@@ -325,6 +325,8 @@ export function loadConfig(env: EnvLike): Config {
   const pushStorePath =
     env['PUSH_STORE_PATH'] || path.join(homeDir, '.web-terminal-push-subs.json')
   const pushMaxSubs = parseNonNegativeInt(env['PUSH_MAX_SUBS'], 'PUSH_MAX_SUBS', DEFAULT_PUSH_MAX_SUBS)
+  const prefsStorePath =
+    env['PREFS_STORE_PATH'] || path.join(homeDir, '.web-terminal-prefs.json')
   const notifyDone = parseBool(env['NOTIFY_DONE'], true)  // default on
   const notifyDnd = parseBool(env['NOTIFY_DND'], false)   // default off (do not disturb)
   // DECISION_TOKEN_TTL_MS defaults to permTimeoutMs (parsed after it, order matters)
@@ -416,6 +418,7 @@ export function loadConfig(env: EnvLike): Config {
     vapidSubject,
     pushStorePath,
     pushMaxSubs,
+    prefsStorePath,
     notifyDone,
     notifyDnd,
     decisionTokenTtlMs,