feat(01-01): add core infrastructure, localization, and Wave 0 tests

- AppDatabase with schemaVersion 1, in-memory executor for testing
- Database provider with @Riverpod(keepAlive: true)
- AppTheme with sage green seed ColorScheme and warm surface overrides
- ThemeNotifier with SharedPreferences persistence, defaults to system
- Full German ARB localization (15 keys) with proper umlauts
- Minimal main.dart with ProviderScope placeholder
- Drift schema v1 captured via make-migrations
- All .g.dart files generated via build_runner
- Wave 0 tests: database (3), color scheme (6), theme (3), localization (2) -- 14 total, all passing

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

lib/l10n/app_de.arb

@@ -1,4 +1,27 @@
 {
   "@@locale": "de",
-  "appTitle": "HouseHoldKeaper"
+  "appTitle": "HouseHoldKeaper",
+  "tabHome": "\u00dcbersicht",
+  "tabRooms": "R\u00e4ume",
+  "tabSettings": "Einstellungen",
+  "homeEmptyTitle": "Noch nichts zu tun!",
+  "homeEmptyMessage": "Lege zuerst einen Raum an, um Aufgaben zu planen.",
+  "homeEmptyAction": "Raum erstellen",
+  "roomsEmptyTitle": "Hier ist noch alles leer!",
+  "roomsEmptyMessage": "Erstelle deinen ersten Raum, um loszulegen.",
+  "roomsEmptyAction": "Raum erstellen",
+  "settingsSectionAppearance": "Darstellung",
+  "settingsThemeLabel": "Farbschema",
+  "themeSystem": "System",
+  "themeLight": "Hell",
+  "themeDark": "Dunkel",
+  "settingsSectionAbout": "\u00dcber",
+  "aboutAppName": "HouseHoldKeaper",
+  "aboutTagline": "Dein Haushalt, entspannt organisiert.",
+  "aboutVersion": "Version {version}",
+  "@aboutVersion": {
+    "placeholders": {
+      "version": { "type": "String" }
+    }
+  }
 }