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/workspace.js

@@ -200,6 +200,7 @@ async function openFile(path){
   $('fileTree').style.display='none';
 
   _previewCurrentPath = path;
+  renderFileBreadcrumb(path);
   if(IMAGE_EXTS.has(ext)){
     // Image: load via raw endpoint, show as <img>
     showPreview('image');
@@ -245,3 +246,41 @@ function downloadFile(path){
   showToast(t('downloading',filename),2000);
 }
 
+
+// ── Render breadcrumb for file preview mode ──────────────────────────────────
+function renderFileBreadcrumb(filePath) {
+  const bar = $('breadcrumbBar');
+  if (!bar) return;
+  bar.style.display = 'flex';
+  const upBtn = $('btnUpDir');
+  if (upBtn) upBtn.style.display = '';
+
+  bar.innerHTML = '';
+  // Root
+  const root = document.createElement('span');
+  root.className = 'breadcrumb-seg breadcrumb-link';
+  root.textContent = '~';
+  root.onclick = () => { clearPreview(); loadDir('.'); };
+  bar.appendChild(root);
+
+  const parts = filePath.split('/');
+  let accumulated = '';
+  for (let i = 0; i < parts.length; i++) {
+    const sep = document.createElement('span');
+    sep.className = 'breadcrumb-sep';
+    sep.textContent = '/';
+    bar.appendChild(sep);
+
+    accumulated += (accumulated ? '/' : '') + parts[i];
+    const seg = document.createElement('span');
+    seg.textContent = parts[i];
+    if (i < parts.length - 1) {
+      seg.className = 'breadcrumb-seg breadcrumb-link';
+      const target = accumulated;
+      seg.onclick = () => { clearPreview(); loadDir(target); };
+    } else {
+      seg.className = 'breadcrumb-seg breadcrumb-current';
+    }
+    bar.appendChild(seg);
+  }
+}