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.

utils/theme.js

@@ -50,8 +50,15 @@ let _navColorTimer = null
 const getThemeById = (id) => THEMES.find(t => t.id === id) || THEMES[0]
 
 const getCurrentTheme = () => {
-  const id = wx.getStorageSync(THEME_KEY) || DEFAULT_THEME_ID
-  return getThemeById(id)
+  let id = wx.getStorageSync(THEME_KEY)
+  if (!id) {
+    // fallback: try recovering from user_settings backup
+    try {
+      const s = wx.getStorageSync('user_settings')
+      if (s && s.themeId) id = s.themeId
+    } catch (e) {}
+  }
+  return getThemeById(id || DEFAULT_THEME_ID)
 }
 
 const _setNavBarColor = (primary) => {
@@ -69,7 +76,17 @@ const _setNavBarColor = (primary) => {
 }
 
 const setTheme = (id) => {
-  wx.setStorageSync(THEME_KEY, id)
+  try {
+    wx.setStorageSync(THEME_KEY, id)
+    // redundant backup in user_settings
+    try {
+      const s = wx.getStorageSync('user_settings') || {}
+      s.themeId = id
+      wx.setStorageSync('user_settings', s)
+    } catch (e) {}
+  } catch (e) {
+    console.error('Failed to save theme:', e)
+  }
   const theme = getThemeById(id)
   _setNavBarColor(theme.primary)
   const app = getApp()