feat(docker): harden session mode + File Viewer button (v1.4.1)

Docker cases: seamless Claude auth (seed ~/.claude.json instead of the
corruption-prone single-file mount), full credential-store isolation for
claude + codex/gemini/gcloud/opencode (share only transcripts/rollouts,
seed the rest), auto-build the base image on first use, C.UTF-8 locale
(fixes box-drawing), collapsed/shortened Create-Case UI + short "(docker)"
case-menu tags, and w<n>-<case> tab naming for docker/remote sessions.
Also: opt-in File Viewer header button; fix a TZ-boundary flaky test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/public/index.html

@@ -124,6 +124,7 @@
           <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="m21.44 11.05-9.19 9.19a6 6 0 0 1-8.49-8.49l9.19-9.19a4 4 0 0 1 5.66 5.66l-9.2 9.19a2 2 0 0 1-2.83-2.83l8.49-8.48"/></svg>
           <span class="attachment-history-badge" id="attachmentHistoryBadge" style="display:none;">0</span>
         </button>
+        <button class="btn-icon-header btn-file-viewer btn-file-viewer--hidden" onclick="app.toggleFileBrowserButton()" title="File Viewer" aria-label="Open file viewer" aria-expanded="false"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M3 7a2 2 0 0 1 2-2h4l2 2h8a2 2 0 0 1 2 2v8a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2z"/></svg></button>
         <button class="btn-icon-header btn-multimonitor btn-multimonitor--hidden" onclick="app.launchMultiMonitor()" title="Open Codeman across all displays" aria-label="Open Codeman across all displays"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="2" y="4" width="13" height="9" rx="1.5"/><rect x="11" y="9" width="11" height="8" rx="1.5"/></svg></button>
         <button class="btn-icon-header btn-ultracode-agents btn-ultracode-agents--hidden" onclick="app.toggleUltracodeAgentsPanel()" title="Ultracode / Workflow agents" aria-label="Open ultracode workflow agents"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><circle cx="6" cy="6" r="2.5"/><circle cx="6" cy="18" r="2.5"/><circle cx="18" cy="12" r="2.5"/><path d="M8.2 7.2 15.6 11M8.2 16.8 15.6 13"/></svg></button>
         <div class="header-plan-usage header-plan-usage--hidden" id="planUsageChip" title="Claude plan usage limits">—</div>
@@ -1268,6 +1269,13 @@
                   <span class="slider"></span>
                 </label>
               </div>
+              <div class="settings-item" title="Show the file viewer button in header (opens the file browser panel for the active session)">
+                <span class="settings-item-label">File Viewer</span>
+                <label class="switch switch-sm">
+                  <input type="checkbox" id="appSettingsShowFileViewerButton">
+                  <span class="slider"></span>
+                </label>
+              </div>
               <div class="settings-item" title="Show the attachments button in header (opens the attachment history drawer)">
                 <span class="settings-item-label">Attachments Button</span>
                 <label class="switch switch-sm">
@@ -1864,12 +1872,12 @@
               <label>Description (optional)</label>
               <input type="text" id="newCaseDescription" placeholder="A brief description..." autocomplete="off">
             </div>
-            <div class="form-row">
-              <label class="checkbox-row"><input type="checkbox" id="newCaseDocker" onchange="app.toggleDockerQuickSettings()"> 🐳 Run in an isolated Docker container</label>
-              <span class="form-hint">One checkbox is enough — it creates the case folder AND a hardened container with default settings, then starts the session inside it. Click to expand for optional presets. Requires the base image (<code>node scripts/build-agent-image.mjs</code>).</span>
+            <div class="form-row docker-quick-row">
+              <label class="checkbox-row"><input type="checkbox" id="newCaseDocker"> 🐳 Run in an isolated Docker container</label>
+              <span class="form-hint">Runs this case in a hardened, isolated container. The base image is built automatically on first use.</span>
             </div>
-            <details class="advanced-options" id="dockerQuickSettings" style="display:none;">
-              <summary>Container settings (optional — sensible defaults)</summary>
+            <details class="advanced-options docker-quick-settings" id="dockerQuickSettings">
+              <summary>Container settings (optional, sensible defaults)</summary>
               <div class="advanced-options-content">
                 <div class="form-row">
                   <label>Template</label>
@@ -1880,7 +1888,7 @@
                     <option value="gpu">GPU — 8 GB RAM, 4 CPU, all GPUs</option>
                     <option value="custom">Custom</option>
                   </select>
-                  <span class="form-hint">Disk is elastic — storage grows automatically as data flows in (no fixed cap).</span>
+                  <span class="form-hint">Disk is elastic: storage grows automatically as data flows in (no fixed cap).</span>
                 </div>
                 <div class="form-row">
                   <label>Memory</label>