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/style.css

@@ -11,7 +11,7 @@
             --inara-border: #3d2a55;
             --accent:       #c4935a;
             --text:         #e8e0f0;
-            --muted:        #9080a8;
+            --muted:        #b0a2c8;
             --error-bg:     #3b0f0f;
             --error-border: #7f1d1d;
             --error-text:   #fca5a5;
@@ -59,7 +59,7 @@
             --inara-border: #3d2a55;
             --accent:       #c4935a;
             --text:         #e8e0f0;
-            --muted:        #9080a8;
+            --muted:        #b0a2c8;
             --error-bg:     #3b0f0f;
             --error-border: #7f1d1d;
             --error-text:   #fca5a5;
@@ -242,6 +242,37 @@
         }
         .session-delete-btn:hover { color: #e06c75; }
 
+        .session-rename-btn {
+            background: none;
+            border: none;
+            color: var(--muted);
+            font-size: 0.9rem;
+            line-height: 1;
+            padding: 2px 6px;
+            cursor: pointer;
+            border-radius: 3px;
+            flex-shrink: 0;
+            opacity: 0.4;
+            transition: opacity 0.15s, color 0.15s;
+            min-width: 24px;
+            text-align: center;
+        }
+        .session-item:hover .session-rename-btn { opacity: 1; }
+        .session-rename-btn:hover { color: var(--accent); }
+
+        .session-rename-input {
+            flex: 1;
+            min-width: 0;
+            background: var(--bg);
+            border: 1px solid var(--accent);
+            border-radius: 4px;
+            color: var(--text);
+            font-family: monospace;
+            font-size: 0.85rem;
+            padding: 1px 5px;
+            outline: none;
+        }
+
         .session-id {
             font-family: monospace;
             font-size: 0.85rem;
@@ -254,7 +285,7 @@
         }
 
         .session-meta {
-            font-size: 0.72rem;
+            font-size: 0.78rem;
             color: var(--muted);
             white-space: nowrap;
             text-align: right;