fix(#363): keep sqlite3 (dart:ffi) out of the web build

The VACUUM INTO snapshot import pulled dart:ffi via package:sqlite3, which
does not compile on web. Isolate it behind a conditional import (native impl
+ web stub); the migration is native-only and never runs on web.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/storage/drift/db_snapshot_io.dart

@@ -0,0 +1,13 @@
+import 'package:sqlite3/sqlite3.dart';
+
+/// Snapshots [source] to [target] with SQLite `VACUUM INTO` (a consistent,
+/// transaction-based copy). Native only; isolated in its own file so the
+/// `dart:ffi`-backed sqlite3 import never reaches the web build (#363).
+void vacuumInto(String source, String target) {
+  final db = sqlite3.open(source, mode: OpenMode.readOnly);
+  try {
+    db.execute("VACUUM INTO '${target.replaceAll("'", "''")}'");
+  } finally {
+    db.close();
+  }
+}

lib/storage/drift/db_snapshot_web.dart

@@ -0,0 +1,6 @@
+/// Web stub for [vacuumInto]. The pinned-directory migration is native-only
+/// (web uses drift's OPFS/IndexedDB backend and never calls this), so this
+/// exists only to keep the conditional import from pulling `dart:ffi` into the
+/// web build (#363).
+void vacuumInto(String source, String target) =>
+    throw UnsupportedError('vacuumInto is native-only');

lib/storage/drift/offband_database.dart

@@ -5,9 +5,11 @@ import 'package:drift_flutter/drift_flutter.dart';
 import 'package:flutter/foundation.dart';
 import 'package:path/path.dart' as p;
 import 'package:path_provider/path_provider.dart';
-import 'package:sqlite3/sqlite3.dart';
 
 import '../../utils/app_logger.dart';
+// sqlite3 uses dart:ffi, which is unavailable on web; import the native impl
+// only off web so `flutter build web` still compiles (#363).
+import 'db_snapshot_web.dart' if (dart.library.io) 'db_snapshot_io.dart';
 
 part 'offband_database.g.dart';
 
@@ -178,12 +180,7 @@ class OffbandDatabase extends _$OffbandDatabase {
   @visibleForTesting
   static void snapshotDatabase(String source, String target) {
     try {
-      final db = sqlite3.open(source, mode: OpenMode.readOnly);
+      vacuumInto(source, target);
-      try {
-        db.execute("VACUUM INTO '${target.replaceAll("'", "''")}'");
-      } finally {
-        db.close();
-      }
       final out = File(target);
       if (!out.existsSync() || out.lengthSync() == 0) {
         throw StateError('VACUUM INTO produced no output at $target');