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/boot.js

@@ -200,6 +200,8 @@ function clearPreview(){
   const pp=$('previewPathText');if(pp)pp.textContent='';
   const ft=$('fileTree');if(ft)ft.style.display='';
   _previewCurrentPath='';_previewCurrentMode='';_previewDirty=false;
+  // Restore directory breadcrumb after closing file preview
+  if(typeof renderBreadcrumb==='function') renderBreadcrumb();
 }
 $('btnClearPreview').onclick=clearPreview;
 // workspacePath click handler removed -- use topbar workspace chip dropdown instead
@@ -302,7 +304,7 @@ document.querySelectorAll('.suggestion').forEach(btn=>{
 // ── Resizable panels ──────────────────────────────────────────────────────
 (function(){
   const SIDEBAR_MIN=180, SIDEBAR_MAX=420;
-  const PANEL_MIN=180,   PANEL_MAX=500;
+  const PANEL_MIN=180,   PANEL_MAX=1200;
 
   function initResize(handleId, targetEl, edge, minW, maxW, storageKey){
     const handle = $(handleId);