fix: legacy sessions (profile=null) leak into all profiles' session lists

Root cause: sessions created before Sprint 22 have no profile tag (profile=None).
The client filter was '!s.profile || s.profile === S.activeProfile' -- the
'!s.profile' guard made ALL 33 legacy sessions visible under every profile,
so switching to Camanji still showed the entire default session history.

Fix:
- api/models.py all_sessions(): backfill profile='default' on sessions with
  no profile tag before returning. This is in-memory only (no disk writes) --
  legacy sessions just get attributed to the default profile at read time.
  Applied to both the index-path and the full-scan fallback path.
- static/sessions.js: tighten the client filter to s.profile === S.activeProfile
  (remove the '!s.profile' escape hatch -- now redundant since server fills it).
  Every session now has an explicit profile, so the filter is precise.

Result: switching to Camanji shows only Camanji sessions. Default profile shows
legacy + default-tagged sessions. 'All profiles' toggle still shows everything.
S.activeProfile defaults to 'default' in the S object so first render is safe.

Tests: 426 passed, 0 failed.

api/models.py

@@ -90,6 +90,11 @@ def all_sessions():
             result = sorted(index_map.values(), key=lambda s: (s.get('pinned', False), s['updated_at']), reverse=True)
             # Hide empty Untitled sessions from the UI (created by tests, page refreshes, etc.)
             result = [s for s in result if not (s.get('title','Untitled')=='Untitled' and s.get('message_count',0)==0)]
+            # Backfill: sessions created before Sprint 22 have no profile tag.
+            # Attribute them to 'default' so the client profile filter works correctly.
+            for s in result:
+                if not s.get('profile'):
+                    s['profile'] = 'default'
             return result
         except Exception:
             pass  # fall through to full scan
@@ -105,7 +110,11 @@ def all_sessions():
     for s in SESSIONS.values():
         if all(s.session_id != x.session_id for x in out): out.append(s)
     out.sort(key=lambda s: (getattr(s, 'pinned', False), s.updated_at), reverse=True)
-    return [s.compact() for s in out if not (s.title=='Untitled' and len(s.messages)==0)]
+    result = [s.compact() for s in out if not (s.title=='Untitled' and len(s.messages)==0)]
+    for s in result:
+        if not s.get('profile'):
+            s['profile'] = 'default'
+    return result
 
 
 def title_from(messages, fallback='Untitled'):