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>

cortex/static/setup.html

@@ -33,7 +33,7 @@
     }
 
     .logo h1 { font-size: 1.6rem; font-weight: 700; letter-spacing: 0.05em; color: #a78bfa; }
-    .logo p  { font-size: 0.8rem; color: #64748b; margin-top: 0.25rem; }
+    .logo p  { font-size: 0.8rem; color: #94a3b8; margin-top: 0.25rem; }
 
     h2 {
       font-size: 1rem;
@@ -52,7 +52,7 @@
       margin-bottom: 0.4rem;
     }
 
-    label small { font-weight: 400; color: #475569; }
+    label small { font-weight: 400; color: #94a3b8; }
 
     input, select {
       width: 100%;
@@ -71,7 +71,7 @@
 
     .field { margin-bottom: 1rem; }
 
-    .hint { font-size: 0.75rem; color: #475569; margin-top: 0.3rem; }
+    .hint { font-size: 0.75rem; color: #94a3b8; margin-top: 0.3rem; }
 
     button[type="submit"] {
       width: 100%;
@@ -98,7 +98,7 @@
 
     .step-label {
       font-size: 0.7rem;
-      color: #475569;
+      color: #94a3b8;
       text-align: right;
       margin-bottom: 1rem;
     }