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/web/server/lib/opencode/settings-helpers.js

@@ -289,6 +289,9 @@ export const createSettingsHelpers = (dependencies) => {
     if (typeof candidate.gitmojiEnabled === 'boolean') {
       result.gitmojiEnabled = candidate.gitmojiEnabled;
     }
+    if (typeof candidate.defaultFileViewerPreview === 'boolean') {
+      result.defaultFileViewerPreview = candidate.defaultFileViewerPreview;
+    }
     if (typeof candidate.zenModel === 'string') {
       const trimmed = candidate.zenModel.trim();
       result.zenModel = trimmed.length > 0 ? trimmed : undefined;