feat(03-01): add daily plan provider with date categorization and localization keys

- dailyPlanProvider: manual StreamProvider.autoDispose with overdue/today/tomorrow partitioning
- Stable progress denominator: remaining overdue + remaining today + completedTodayCount
- 10 new German localization keys for daily plan sections, progress, empty states
- dart analyze clean, full test suite (66/66) passes

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

lib/l10n/app_de.arb

@@ -68,5 +68,25 @@
     "placeholders": {
       "count": { "type": "int" }
     }
-  }
+  },
+  "dailyPlanProgress": "{completed} von {total} erledigt",
+  "@dailyPlanProgress": {
+    "placeholders": {
+      "completed": { "type": "int" },
+      "total": { "type": "int" }
+    }
+  },
+  "dailyPlanSectionOverdue": "\u00dcberf\u00e4llig",
+  "dailyPlanSectionToday": "Heute",
+  "dailyPlanSectionUpcoming": "Demn\u00e4chst",
+  "dailyPlanUpcomingCount": "Demn\u00e4chst ({count})",
+  "@dailyPlanUpcomingCount": {
+    "placeholders": {
+      "count": { "type": "int" }
+    }
+  },
+  "dailyPlanAllClearTitle": "Alles erledigt! \ud83c\udf1f",
+  "dailyPlanAllClearMessage": "Keine Aufgaben f\u00fcr heute. Genie\u00dfe den Moment!",
+  "dailyPlanNoOverdue": "Keine \u00fcberf\u00e4lligen Aufgaben",
+  "dailyPlanNoTasks": "Noch keine Aufgaben angelegt"
 }