feat(settings): reorder top-level settings sections and add appearance subtext (#169)

Reorders SettingsScreen's top-level (non-Advanced) sections per #158:
Accounts, Message downloading, Contacts, Appearance, Settings Backup,
Notifications, Storage on this device, then a header-less trailing
Report a Problem row (mirrors AccountSettingsScreen's headerless
"Remove account" row now that Diagnostics is down to one item).

- Move "Message downloading" up to directly follow Accounts.
- Add a two-line descriptive subtext under the Appearance header
  ("Match device theme" / "Material You theming (Android 12+)"); no
  new toggle, since LibreMailTheme already always follows the system
  light/dark setting via isSystemInDarkTheme().
- Rename settings_backup "Backup" -> "Settings Backup" and
  settings_new_mail "New-mail notifications" -> "New mail
  notifications" (drop hyphen).
- Drop the now-single-item settings_diagnostics header string; keep
  Contacts positioned directly before Appearance (its prior relative
  spot), per the ticket's suggested safest default for the
  unresolved placement question.

Advanced's internal order is untouched (out of scope; tracked by
#162).

Closes #158

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/res/values/strings.xml

@@ -224,13 +224,13 @@
     <string name="fetch_on_demand">Always on-demand</string>
     <string name="fetch_on_demand_summary">Sync only message headers; download a message\'s content when you open it.</string>
     <string name="settings_notifications">Notifications</string>
-    <string name="settings_new_mail">New-mail notifications</string>
+    <string name="settings_new_mail">New mail notifications</string>
     <string name="settings_new_mail_summary">Show a notification when new mail arrives</string>
     <string name="settings_no_accounts">No accounts yet</string>
     <string name="account_remove">Remove account</string>
 
     <!-- Backup -->
-    <string name="settings_backup">Backup</string>
+    <string name="settings_backup">Settings Backup</string>
     <string name="settings_backup_include">Include settings in Android Backup</string>
     <string name="settings_backup_include_summary">Let Android back up your LibreMail preferences (Google Auto Backup) so they restore when you set up a new device. Your mail, accounts, passwords, and encryption keys are never backed up — only app settings. Off by default; uses Google infrastructure.</string>
 
@@ -278,6 +278,7 @@
     <string name="settings_account_system_notif">Sound &amp; vibration</string>
     <string name="settings_account_system_notif_summary">Manage sound, vibration, and importance in Android settings</string>
     <string name="settings_appearance">Appearance</string>
+    <string name="settings_appearance_summary">Match device theme\nMaterial You theming (Android 12+)</string>
     <string name="settings_dynamic_color">Use wallpaper colors</string>
     <string name="settings_dynamic_color_summary">Material You dynamic theming (Android 12+)</string>
     <string name="settings_add_account">Add account</string>
@@ -319,7 +320,6 @@
     <string name="settings_contacts_open_settings">Open settings</string>
 
     <!-- Diagnostics / debug reporting -->
-    <string name="settings_diagnostics">Diagnostics</string>
     <string name="settings_report_problem">Report a problem</string>
     <string name="settings_report_problem_summary">Capture diagnostics you can review and optionally send</string>
     <string name="reports_title">Problem reports</string>