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/app.js

@@ -235,8 +235,12 @@
             }
         });
 
+        // session_id → friendly name (populated on each panel render)
+        const sessionNames = new Map();
+
         function renderPanel(sessions) {
             sessionsPanel.innerHTML = '';
+            sessionNames.clear();
 
             const newItem = makeItem('new', '+ New session', '');
             newItem.addEventListener('click', () => {
@@ -259,13 +263,57 @@
             }
 
             for (const s of sessions) {
+                const displayName = s.name || s.session_id;
+                sessionNames.set(s.session_id, displayName);
+
                 const item = makeItem(
                     s.session_id === sessionId ? 'active' : '',
-                    s.session_id,
+                    displayName,
                     `${s.message_count} msgs · ${timeAgo(s.updated)}`
                 );
                 item.addEventListener('click', () => resumeSession(s.session_id));
 
+                // Rename button (✎)
+                const renameBtn = document.createElement('button');
+                renameBtn.className = 'session-rename-btn';
+                renameBtn.textContent = '✎';
+                renameBtn.title = 'Rename session';
+                renameBtn.addEventListener('click', async (e) => {
+                    e.stopPropagation();
+                    const labelEl = item.querySelector('.session-id');
+                    const current = s.name || '';
+                    const input = document.createElement('input');
+                    input.className = 'session-rename-input';
+                    input.value = current;
+                    input.placeholder = s.session_id;
+                    labelEl.replaceWith(input);
+                    input.focus();
+                    input.select();
+
+                    async function commitRename() {
+                        const newName = input.value.trim();
+                        await fetch(`/sessions/${s.session_id}?${_fileParams}`, {
+                            method: 'PATCH',
+                            headers: { 'Content-Type': 'application/json' },
+                            body: JSON.stringify({ name: newName }),
+                        });
+                        const res = await fetch(`/sessions?${_fileParams}`);
+                        const data = await res.json();
+                        renderPanel(data.sessions);
+                        // Update status bar if this is the active session
+                        if (sessionId === s.session_id) {
+                            sessionEl.textContent = `session: ${newName || s.session_id}`;
+                        }
+                    }
+
+                    input.addEventListener('keydown', (e) => {
+                        if (e.key === 'Enter') { e.preventDefault(); commitRename(); }
+                        if (e.key === 'Escape') { renderPanel(sessions); }
+                    });
+                    input.addEventListener('blur', commitRename);
+                });
+                item.appendChild(renameBtn);
+
                 const delBtn = document.createElement('button');
                 delBtn.className = 'session-delete-btn';
                 delBtn.textContent = '×';
@@ -316,7 +364,7 @@
 
             messagesEl.innerHTML = '';
             sessionId = id;
-            sessionEl.textContent = `session: ${id}`;
+            sessionEl.textContent = `session: ${sessionNames.get(id) || id}`;
             currentHistory = [];
 
             for (let i = 0; i < data.messages.length; i++) {