feat: add 'Open files in preview mode' setting (#988)

* feat: add 'Open files in preview mode' setting

- Add defaultFileViewerPreview setting to persist user preference for file viewer default mode
- Add checkbox in Settings → Sessions → Session Defaults to toggle the setting
- Files now open in preview mode by default when setting is enabled
- Respects per-file-type localStorage persistence for markdown/HTML/JSON files
- Setting persists across sessions via /api/config/settings endpoint

* fix: address PR review feedback

- FilesView: respect HTML localStorage preference in file-change effect,
  falling back to global setting only when nothing stored
- DefaultsSettings: read defaultFileViewerPreview directly from config store
  instead of redundant local state + separate fetch
- DefaultsSettings: extract duplicated toggle logic into
  handleToggleFileViewerPreview callback

* fix: honor default preview setting for markdown files

Previously mdViewMode only read localStorage on mount (deps: []), so
when settingsDefaultFileViewerPreview was enabled and no MD_VIEWER_MODE_KEY
was stored, markdown files silently opened in edit mode — ignoring the
setting for the very file type users most want to preview.

Fold md init into the per-file-change effect, mirroring the html handling:
localStorage preference wins, falling back to the setting-derived default.
The saveMdViewMode callback stays untouched so user-initiated toggles
still persist.

---------

Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/src/components/sections/openchamber/DefaultsSettings.tsx

@@ -30,6 +30,8 @@ export const DefaultsSettings: React.FC = () => {
   const setSettingsDefaultModel = useConfigStore((state) => state.setSettingsDefaultModel);
   const setSettingsDefaultVariant = useConfigStore((state) => state.setSettingsDefaultVariant);
   const setSettingsDefaultAgent = useConfigStore((state) => state.setSettingsDefaultAgent);
+  const setSettingsDefaultFileViewerPreview = useConfigStore((state) => state.setSettingsDefaultFileViewerPreview);
+  const settingsDefaultFileViewerPreview = useConfigStore((state) => state.settingsDefaultFileViewerPreview);
   const showDeletionDialog = useUIStore((state) => state.showDeletionDialog);
   const setShowDeletionDialog = useUIStore((state) => state.setShowDeletionDialog);
   const providers = useConfigStore((state) => state.providers);
@@ -187,6 +189,12 @@ export const DefaultsSettings: React.FC = () => {
     [setAgent, setSettingsDefaultAgent]
   );
 
+  const handleToggleFileViewerPreview = React.useCallback(() => {
+    const next = !settingsDefaultFileViewerPreview;
+    setSettingsDefaultFileViewerPreview(next);
+    updateDesktopSettings({ defaultFileViewerPreview: next }).catch(console.warn);
+  }, [settingsDefaultFileViewerPreview, setSettingsDefaultFileViewerPreview]);
+
   const availableVariants = React.useMemo(() => {
     if (!parsedModel.providerId || !parsedModel.modelId) return [];
     const provider = providers.find((p) => p.id === parsedModel.providerId);
@@ -300,6 +308,23 @@ export const DefaultsSettings: React.FC = () => {
           <span className="typography-ui-label text-foreground">Show Deletion Dialog</span>
         </div>
 
+        <div
+          className="group flex cursor-pointer items-center gap-2 py-1"
+          role="button"
+          tabIndex={0}
+          aria-pressed={settingsDefaultFileViewerPreview}
+          onClick={handleToggleFileViewerPreview}
+          onKeyDown={(event) => {
+            if (event.key === ' ' || event.key === 'Enter') {
+              event.preventDefault();
+              handleToggleFileViewerPreview();
+            }
+          }}
+        >
+          <Checkbox checked={settingsDefaultFileViewerPreview} onChange={setSettingsDefaultFileViewerPreview} ariaLabel="Open files in preview mode" />
+          <span className="typography-ui-label text-foreground">Open files in preview mode</span>
+        </div>
+
       </section>
     </div>
   );