feat(02-03): create task providers, form screen with frequency and effort selectors

- TaskActions AsyncNotifier for create, update, delete, complete task mutations
- tasksInRoomProvider manual StreamProvider.family wrapping TasksDao.watchTasksInRoom
- TaskFormScreen with name, frequency (10 presets + custom), effort (3-way segmented),
  description, and initial due date picker (German DD.MM.YYYY format)
- Custom frequency: number + unit picker (Tage/Wochen/Monate)
- Calendar-anchored intervals auto-set anchorDay from due date
- Edit mode loads existing task and pre-fills all fields
- 19 new German localization keys for task form, delete, and empty state

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

lib/l10n/app_de.arb

@@ -39,5 +39,25 @@
       "count": { "type": "int" }
     }
   },
-  "cancel": "Abbrechen"
+  "cancel": "Abbrechen",
+  "taskFormCreateTitle": "Aufgabe erstellen",
+  "taskFormEditTitle": "Aufgabe bearbeiten",
+  "taskFormNameLabel": "Aufgabenname",
+  "taskFormNameHint": "z.B. Staubsaugen, Fenster putzen...",
+  "taskFormNameRequired": "Bitte einen Namen eingeben",
+  "taskFormFrequencyLabel": "Wiederholung",
+  "taskFormFrequencyCustom": "Benutzerdefiniert",
+  "taskFormFrequencyEvery": "Alle",
+  "taskFormFrequencyUnitDays": "Tage",
+  "taskFormFrequencyUnitWeeks": "Wochen",
+  "taskFormFrequencyUnitMonths": "Monate",
+  "taskFormEffortLabel": "Aufwand",
+  "taskFormDescriptionLabel": "Beschreibung (optional)",
+  "taskFormDueDateLabel": "Erstes F\u00e4lligkeitsdatum",
+  "taskDeleteConfirmTitle": "Aufgabe l\u00f6schen?",
+  "taskDeleteConfirmMessage": "Die Aufgabe wird unwiderruflich gel\u00f6scht.",
+  "taskDeleteConfirmAction": "L\u00f6schen",
+  "taskEmptyTitle": "Noch keine Aufgaben",
+  "taskEmptyMessage": "Erstelle die erste Aufgabe f\u00fcr diesen Raum.",
+  "taskEmptyAction": "Aufgabe erstellen"
 }