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/streaming.py

@@ -334,7 +334,9 @@ def _run_agent_streaming(session_id, msg_text, model, workspace, stream_id, atta
             for _m in s.messages:
                 if isinstance(_m, dict) and not _m.get('timestamp') and not _m.get('_ts'):
                     _m['timestamp'] = int(_now)
-            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)
             # Read token/cost usage from the agent object (if available)
             input_tokens = getattr(agent, 'session_prompt_tokens', 0) or 0
             output_tokens = getattr(agent, 'session_completion_tokens', 0) or 0