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.

static/sessions.js

@@ -115,7 +115,9 @@ function renderSessionListFromCache(){
   const titleIds=new Set(titleMatches.map(s=>s.session_id));
   const allMatched=q?[...titleMatches,..._contentSearchResults.filter(s=>!titleIds.has(s.session_id))]:titleMatches;
   // Filter by active profile (unless "All profiles" is toggled on)
-  const profileFiltered=_showAllProfiles?allMatched:allMatched.filter(s=>!s.profile||s.profile===S.activeProfile);
+  // Server backfills profile='default' for legacy sessions, so every session has a profile.
+  // Show only sessions tagged to the active profile; 'All profiles' toggle overrides.
+  const profileFiltered=_showAllProfiles?allMatched:allMatched.filter(s=>s.profile===S.activeProfile);
   // Filter by active project
   const projectFiltered=_activeProject?profileFiltered.filter(s=>s.project_id===_activeProject):profileFiltered;
   // Filter archived unless toggle is on