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>

static/style.css

@@ -322,7 +322,9 @@
   .chip{font-size:11px;padding:4px 10px;border-radius:999px;background:rgba(255,255,255,0.05);border:1px solid var(--border2);color:var(--muted);font-weight:500;}
   .chip.model{color:var(--blue);border-color:rgba(124,185,255,0.35);background:rgba(124,185,255,0.1);}
   .messages{flex:1;overflow-y:auto;display:flex;flex-direction:column;min-height:0;position:relative;z-index:0;}
-  .messages-inner{max-width:800px;margin:0 auto;width:100%;padding:20px 24px 32px;display:flex;flex-direction:column;}
+  .messages-inner{margin:0 auto;width:100%;padding:20px 24px 32px;display:flex;flex-direction:column;}
+  @media(min-width:1400px){.messages-inner{max-width:1100px;}}
+  @media(min-width:1800px){.messages-inner{max-width:1200px;}}
   .msg-row{padding:10px 0;}
   .msg-row+.msg-row{border-top:none;}
   .msg-role{font-size:12px;font-weight:500;letter-spacing:.01em;margin-bottom:8px;display:flex;align-items:center;gap:8px;}