feat: Schlagwörter je Kategorie + auf-/zuklappbares Kategorie-Dropdown

- Manuelle Schlagwörter je Kategorie (Popup mit Chips, Doppelklick zum Bearbeiten)
- Treffer über Empfänger ODER Verwendungszweck; längstes Schlagwort gewinnt;
  Kind-Filter (Ausgabe/Einnahme); wirken nur bei künftigen Importen
- Lernende Korrektur-Erkennung: ab 5 Anwendungen und >= 1/3 Korrekturen
  erscheint eine Prüf-Meldung in den Einstellungen (Behalten/Entfernen)
- Neue CategorySelect-Komponente (Baum + Suche, per Portal) ersetzt die
  nativen optgroup-Selects in Buchungen und Import-Prüfansicht
- Migration 0006 (source/applied_count/overridden_count/confirmed, match_field any)
- Endpunkte, i18n DE/EN, Tests (test_keywords: 7 neu, gesamt 27 grün)

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

web/src/locales/en.json

@@ -79,7 +79,12 @@
     "exportTxnJson": "Transactions (JSON)",
     "exportAll": "All data (JSON)",
     "dbBackup": "Download full DB backup",
-    "dbBackupHint": "Complete database of all users (admin only) — keep it for restore."
+    "dbBackupHint": "Complete database of all users (admin only) — keep it for restore.",
+    "keywordReview": "Review keywords",
+    "keywordReviewHint": "These keywords keep getting corrected away — maybe they no longer fit?",
+    "keywordCorrected": "{{overridden}} of {{applied}} transactions corrected",
+    "keep": "Keep",
+    "remove": "Remove"
   },
   "dashboard": {
     "title": "Overview",
@@ -153,7 +158,14 @@
     "expense": "Expense",
     "emptyGroup": "No categories in this group.",
     "deleteConfirm": "Delete this category?",
-    "deleteGroupConfirm": "Delete this group and all its categories?"
+    "deleteGroupConfirm": "Delete this group and all its categories?",
+    "keywordsBtn": "Keywords",
+    "keywordsTitle": "Keywords – {{name}}",
+    "keywordsHint": "Transactions whose payee or description contains one of these words are automatically assigned to this category on future imports.",
+    "keywordAdd": "Type a word and press Enter",
+    "keywordEmpty": "No keywords yet.",
+    "keywordEditHint": "Double-click a keyword to edit it.",
+    "searchPlaceholder": "Search category …"
   },
   "transactions": {
     "title": "Transactions",