feat: session naming, username/persona rename, help page, contrast fixes

- Session name field: PATCH /sessions/{id} endpoint, inline rename button in UI
- Persona rename: inline ✏ toggle form in settings, POST /settings/persona/rename
- Username rename: inline form in settings, POST /settings/username (renames home dir, forces re-login)
- Help page: dedicated /help route replacing modal, collapsible sections
- Per-persona isolation: files.py and session_store.py now scope to correct user/persona
- Contrast/visibility: muted text bumped to slate-400+, session rename btn at 0.4 opacity

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

home/scott/persona/inara/TASKS.json

@@ -25,5 +25,14 @@
     "priority": "normal",
     "created_at": "2026-03-20T05:14:37.344354+00:00",
     "updated_at": "2026-03-20T05:14:37.344368+00:00"
+  },
+  {
+    "id": "t_ODYEciPP",
+    "title": "Check green laser pointers",
+    "description": "Some time this coming weekend.",
+    "status": "todo",
+    "priority": "normal",
+    "created_at": "2026-03-24T02:20:33.546765+00:00",
+    "updated_at": "2026-03-24T02:20:33.546782+00:00"
   }
 ]