feat(layout): unified app-shell primitives (Phase A, no consumer changes)

Build the shared building blocks for the unified bottom-nav UX across the
hub + 7 standalones. Phase A is groundwork only — no App.vue or Hub.vue
consumer is wired up yet, so this commit is purely additive.

New components in src/components/layout/:
- PreferencesRow.vue   theme/language/currency triad (row + list layouts)
- ProfileSheetContent.vue  identity card + back-to-hub + prefs + ProfileSettings
- ProfileSheetTrigger.vue  bottom-row Profile button → opens sheet
- HubPill.vue          fixed top-right back-to-hub link
- BottomNav.vue        consumer tabs + appended Profile slot
- AppShell.vue         outer wrapper composing the above

New composable: useCurrentUserAvatar — picture/displayName/fallbackInitial
from the auth user object.

i18n: new common.nav.* namespace in en/es/fr (typed via LocaleMessages).

Env: VITE_HUB_ROOT_URL added to .env.example with path/subdomain/local
guidance — consumed by HubPill and the back-to-hub sheet item.

Phase B (consumer refactor: chat/wallet/tasks first, then forum/libra/
market/activities, then hub) lands separately.

.env.example

@@ -87,6 +87,25 @@ VITE_HUB_FORUM_URL=
 VITE_HUB_MARKET_URL=
 VITE_HUB_TASKS_URL=
 
+# ───────────────────────────────────────────────────────────────────────
+# VITE_HUB_ROOT_URL — standalone → hub (the inverse of the URLs above)
+#
+# Read by the standalone shell's <HubPill> (top-right "back to hub" link)
+# and the "Back to Hub" item inside the profile sheet. Each standalone's
+# bundle gets this value baked in at build time.
+#
+# In PATH-MODE deployment the standalone and hub share an origin, so the
+# default ('/' if unset) is correct — the link is same-origin.
+#
+# In SUBDOMAIN-MODE production the standalone is on a different origin
+# than the hub; set this to the full hub URL:
+#   VITE_HUB_ROOT_URL=https://app.example.com/
+#
+# In LOCAL DEV with `npm run dev:all`, the hub is on :5173:
+#   VITE_HUB_ROOT_URL=http://localhost:5173/
+# ───────────────────────────────────────────────────────────────────────
+VITE_HUB_ROOT_URL=
+
 # ───────────────────────────────────────────────────────────────────────
 # VITE_BASE_PATH — build-time only, NOT per .env
 #