fix+feat: session title guard + breadcrumb nav + wider panel + responsive msgs (closes #300, #292)

PR #301 changes:
- api/streaming.py: guard title_from() with s.title == 'Untitled' check
- api/routes.py: same guard in sync/non-streaming path

PR #302 changes (cleaned — restores accidentally-removed features):
- static/boot.js: PANEL_MAX 500 -> 1200
- static/boot.js: clearPreview() calls renderBreadcrumb() to restore dir view
- static/style.css: responsive .messages-inner breakpoints (1400px/1800px)
- static/workspace.js: renderFileBreadcrumb() function with clickable segments
- static/workspace.js: openFile() calls renderFileBreadcrumb(path)

12 new tests in tests/test_sprint35.py

Note: PR #302 branch contained several accidental regressions (removed app-dialog
system, onboarding CSS, _checkProviderMismatch, closeMobileFiles, etc.) that were
not part of its stated scope. This clean branch applies only the three intended
features on top of current master.

Co-authored-by: Nathan Esquenazi <nesquena@gmail.com>

api/routes.py

@@ -1543,7 +1543,9 @@ def _handle_chat_sync(handler, body):
             else:
                 os.environ["HERMES_SESSION_KEY"] = old_session_key
     s.messages = result.get("messages") or s.messages
-    s.title = title_from(s.messages, s.title)
+    # Only auto-generate title when still default; preserves user renames
+    if s.title == "Untitled":
+        s.title = title_from(s.messages, s.title)
     s.save()
     # Sync to state.db for /insights (opt-in setting)
     try: