fix: theme persistence — error handling, backup, stale highlight

Bug 1 (critical): setTheme lacked try/catch around wx.setStorageSync.
  If storage write throws (full / permission), the theme silently
  fails to persist but UI has already changed → on next cold start
  getCurrentTheme falls back to orange.
  → Wrap in try/catch; still apply UI change even if storage fails
    (better to see the right color now than revert immediately).

Bug 2 (display): settings onShow called applyThemeToPage (updates
  'theme' object) but never refreshed 'currentThemeId'. After tab
  switching, the radio highlight showed the wrong theme.
  → Add setData({ currentThemeId }) in onShow.

Redundancy: theme now also saved as themeId in user_settings.
  getCurrentTheme falls back to user_settings.themeId if app_theme
  key is unexpectedly missing, providing a recovery path.

pages/settings/settings.js

@@ -39,6 +39,8 @@ Page({
       if (tb) tb.setData({ selected: 2 })
     } catch (e) {}
     themeMod.applyThemeToPage(this)
+    // sync highlighted theme in case it was changed elsewhere
+    this.setData({ currentThemeId: themeMod.getCurrentTheme().id })
   },
 
   onSelectTheme(e) {