feat(backup): add pre-migration backup, periodic backup, backfill warning, and backup management UI

Four improvements to the database backup mechanism:

1. Auto backup before schema migration - creates a snapshot when
   upgrading from an older database version, providing a safety net
   beyond the existing SAVEPOINT rollback mechanism.

2. Periodic startup backup - checks on app launch whether the latest
   backup is older than 24 hours and creates a new one if needed,
   ensuring all users have recent backups regardless of usage patterns.

3. Backfill failure notification - switch now returns SwitchResult with
   warnings instead of silently ignoring backfill errors, so users are
   informed when their manual config changes may not have been saved.

4. Backup management UI - new BackupListSection in Settings > Data
   Management showing all backup snapshots with restore capability,
   including a confirmation dialog and automatic safety backup before
   restore.

src-tauri/src/commands/provider.rs

@@ -4,7 +4,9 @@ use tauri::State;
 use crate::app_config::AppType;
 use crate::error::AppError;
 use crate::provider::Provider;
-use crate::services::{EndpointLatency, ProviderService, ProviderSortUpdate, SpeedtestService};
+use crate::services::{
+    EndpointLatency, ProviderService, ProviderSortUpdate, SpeedtestService, SwitchResult,
+};
 use crate::store::AppState;
 use std::str::FromStr;
 
@@ -67,7 +69,11 @@ pub fn remove_provider_from_live_config(
         .map_err(|e| e.to_string())
 }
 
-fn switch_provider_internal(state: &AppState, app_type: AppType, id: &str) -> Result<(), AppError> {
+fn switch_provider_internal(
+    state: &AppState,
+    app_type: AppType,
+    id: &str,
+) -> Result<SwitchResult, AppError> {
     ProviderService::switch(state, app_type, id)
 }
 
@@ -76,7 +82,7 @@ pub fn switch_provider_test_hook(
     state: &AppState,
     app_type: AppType,
     id: &str,
-) -> Result<(), AppError> {
+) -> Result<SwitchResult, AppError> {
     switch_provider_internal(state, app_type, id)
 }
 
@@ -85,11 +91,9 @@ pub fn switch_provider(
     state: State<'_, AppState>,
     app: String,
     id: String,
-) -> Result<bool, String> {
+) -> Result<SwitchResult, String> {
     let app_type = AppType::from_str(&app).map_err(|e| e.to_string())?;
-    switch_provider_internal(&state, app_type, &id)
-        .map(|_| true)
-        .map_err(|e| e.to_string())
+    switch_provider_internal(&state, app_type, &id).map_err(|e| e.to_string())
 }
 
 fn import_default_config_internal(state: &AppState, app_type: AppType) -> Result<bool, AppError> {