feat(sync-log): add per-mailbox timing to sync log (#104)

Track how long each mailbox takes to sync and display it in the
sync log expanded view (e.g. "2 new · 5 up-to-date · 1.3s").

- Add optional `duration` field to `MailboxSyncStats`
- Capture per-mailbox start/end time in both IMAP and JMAP sync loops
- Store as `duration_ms` in `sync_log_mailboxes` (schema v30 migration)
- Read back and reconstruct `Duration` in repository
- Show timing alongside fetch/skip counts in per-mailbox breakdown
- Extract `_fmtDuration` helper, reuse for the existing total duration

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/data/db/database.dart

@@ -204,6 +204,8 @@ class SyncLogMailboxes extends Table {
   IntColumn get fetched => integer().withDefault(const Constant(0))();
   IntColumn get skipped => integer().withDefault(const Constant(0))();
   IntColumn get bytesTransferred => integer().withDefault(const Constant(0))();
+  // Added in schema v30: how long this mailbox took to sync, in milliseconds.
+  IntColumn get durationMs => integer().nullable()();
 }
 
 /// Stores the result of the periodic "ground truth" verification.
@@ -290,7 +292,7 @@ class AppDatabase extends _$AppDatabase {
   AppDatabase([QueryExecutor? executor]) : super(executor ?? _openConnection());
 
   @override
-  int get schemaVersion => 29;
+  int get schemaVersion => 30;
 
   Future<void> _createEmailFts() async {
     await customStatement('''
@@ -522,6 +524,9 @@ class AppDatabase extends _$AppDatabase {
           if (from < 29) {
             await m.createTable(localSieveScripts);
           }
+          if (from >= 12 && from < 30) {
+            await m.addColumn(syncLogMailboxes, syncLogMailboxes.durationMs);
+          }
         },
       );
 }

lib/data/repositories/sync_log_repository_impl.dart

@@ -49,6 +49,7 @@ class SyncLogRepositoryImpl implements SyncLogRepository {
                 fetched: Value(s.fetched),
                 skipped: Value(s.skipped),
                 bytesTransferred: Value(s.bytesTransferred),
+                durationMs: Value(s.duration?.inMilliseconds),
               ),
             );
       }
@@ -90,6 +91,9 @@ class SyncLogRepositoryImpl implements SyncLogRepository {
                     fetched: m.fetched,
                     skipped: m.skipped,
                     bytesTransferred: m.bytesTransferred,
+                    duration: m.durationMs != null
+                        ? Duration(milliseconds: m.durationMs!)
+                        : null,
                   ),
                 )
                 .toList(),