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/session_store.py

@@ -62,12 +62,29 @@ def save(session_id: str, messages: list[dict]) -> None:
     # Enforce rolling window
     windowed = messages[-settings.max_history_messages:]
 
-    path.write_text(json.dumps({
+    data = {
         "session_id": session_id,
         "created": existing.get("created", datetime.now().isoformat()),
         "updated": datetime.now().isoformat(),
         "messages": windowed,
-    }, indent=2))
+    }
+    if "name" in existing:
+        data["name"] = existing["name"]
+    path.write_text(json.dumps(data, indent=2))
+
+
+def rename(session_id: str, name: str) -> bool:
+    """Set (or clear) the friendly name on a session. Returns False if not found."""
+    path = _path(session_id)
+    if not path.exists():
+        return False
+    data = json.loads(path.read_text())
+    if name:
+        data["name"] = name
+    else:
+        data.pop("name", None)
+    path.write_text(json.dumps(data, indent=2))
+    return True
 
 
 def delete(session_id: str) -> bool:
@@ -87,11 +104,13 @@ def list_all() -> list[dict]:
     for f in sorted(d.glob("*.json"), key=lambda p: p.stat().st_mtime, reverse=True):
         try:
             data = json.loads(f.read_text())
-            results.append({
+            entry = {
                 "session_id": data["session_id"],
+                "name": data.get("name", ""),
                 "updated": data.get("updated"),
                 "message_count": len(data.get("messages", [])),
-            })
+            }
+            results.append(entry)
         except Exception:
             pass
     return results