settings: Make general bool global settings, so as to add without migrations

This new table to read looks to add something like 5-8ms to the app's
startup time.  Not nothing; but not a big cost compared to the time
we spend loading server data right afterward.

Specifically, results on my Pixel 8 (compare to those in the
previous commit):

db load time 117.9ms total: 15.2ms init, 89.2ms settings, 5.1ms bool-settings, 8.3ms accounts
db load time 90.9ms total: 1.3ms init, 78.5ms settings, 8.3ms bool-settings, 2.8ms accounts
db load time 87.2ms total: 1.4ms init, 71.8ms settings, 5.9ms bool-settings, 8.1ms accounts
db load time 85.7ms total: 1.2ms init, 72.8ms settings, 4.9ms bool-settings, 6.8ms accounts
db load time 91.3ms total: 1.4ms init, 80.0ms settings, 7.5ms bool-settings, 2.5ms accounts
db load time 83.8ms total: 1.1ms init, 70.0ms settings, 5.0ms bool-settings, 7.7ms accounts

Author: gnprice

lib/model/database.dart

@@ -23,6 +23,44 @@ class GlobalSettings extends Table {
 
   Column<String> get browserPreference => textEnum<BrowserPreference>()
     .nullable()();
+
+  // If adding a new column to this table, consider whether [BoolGlobalSettings]
+  // can do the job instead (by adding a value to the [BoolGlobalSetting] enum).
+  // That way is more convenient, when it works, because
+  // it avoids a migration and therefore several added copies of our schema
+  // in the Drift generated files.
+}
+
+/// The table of the user's bool-valued, account-independent settings.
+///
+/// These apply across all the user's accounts on this client
+/// (i.e. on this install of the app on this device).
+///
+/// Each row is a [BoolGlobalSettingRow],
+/// referring to a possible setting from [BoolGlobalSetting].
+/// For settings in [BoolGlobalSetting] without a row in this table,
+/// the setting's value is that of [BoolGlobalSetting.default_].
+@DataClassName('BoolGlobalSettingRow')
+class BoolGlobalSettings extends Table {
+  /// The setting's name, a possible name from [BoolGlobalSetting].
+  ///
+  /// The table may have rows where [name] is not the name of any
+  /// enum value in [BoolGlobalSetting].
+  /// This happens if the app has previously run at a future or modified
+  /// version which had additional values in that enum,
+  /// and the user set one of those additional settings.
+  /// The app ignores any such unknown rows.
+  Column<String> get name => text()();
+
+  /// The user's chosen value for the setting.
+  ///
+  /// This is non-nullable; if the user wants to revert to
+  /// following the app's default for the setting,
+  /// that can be expressed by deleting the row.
+  Column<bool> get value => boolean()();
+
+  @override
+  Set<Column<Object>>? get primaryKey => {name};
 }
 
 /// The table of [Account] records in the app's database.
@@ -82,12 +120,14 @@ VersionedSchema _getSchema({
       return Schema4(database: database);
     case 5:
       return Schema5(database: database);
+    case 6:
+      return Schema6(database: database);
     default:
       throw Exception('unknown schema version: $schemaVersion');
   }
 }
 
-@DriftDatabase(tables: [GlobalSettings, Accounts])
+@DriftDatabase(tables: [GlobalSettings, BoolGlobalSettings, Accounts])
 class AppDatabase extends _$AppDatabase {
   AppDatabase(super.e);
 
@@ -102,7 +142,7 @@ class AppDatabase extends _$AppDatabase {
   //  * Write a migration in `_migrationSteps` below.
   //  * Write tests.
   @override
-  int get schemaVersion => 5; // See note.
+  int get schemaVersion => 6; // See note.
 
   static Future<void> _dropAndCreateAll(Migrator m, {
     required int schemaVersion,
@@ -156,6 +196,9 @@ class AppDatabase extends _$AppDatabase {
         }
       });
     },
+    from5To6: (m, schema) async {
+      await m.createTable(schema.boolGlobalSettings);
+    },
   );
 
   @override
@@ -186,6 +229,17 @@ class AppDatabase extends _$AppDatabase {
     return await (select(globalSettings)..limit(1)).getSingle();
   }
 
+  Future<Map<BoolGlobalSetting, bool>> getBoolGlobalSettings() async {
+    final result = <BoolGlobalSetting, bool>{};
+    final rows = await select(boolGlobalSettings).get();
+    for (final row in rows) {
+      final setting = BoolGlobalSetting.byName(row.name);
+      if (setting == null) continue;
+      result[setting] = row.value;
+    }
+    return result;
+  }
+
   Future<int> createAccount(AccountsCompanion values) async {
     try {
       return await into(accounts).insert(values);