Fix reminder dialog sizing, item editing, and old-repo fee defaults

The dialog's base class sets resizable(False, False) before this
subclass's content (including the preset-populated items table) exists,
so its initial size stayed locked to a too-small guess and cut off the
bottom. It now explicitly sizes to its actual content after everything,
including the selected preset's items, has been built.

The items table only supported add/remove -- there was no way to change
an already-added row's amount (e.g. after picking the Rücklastschrift
preset, its prefilled fee couldn't be adjusted). Selecting a row now
loads it into the description/amount fields, and a new "Aktualisieren"
button applies edits back to that row.

The Rücklastschrift preset label was missing the "Stufe N:" prefix the
other presets have, inconsistent for no reason.

Also fixed a real gap: repositories created before standard_fee_items
existed had no such key in repository.json at all, so Optionen showed
an empty Standardpositionen table instead of the built-in defaults.
get_reminder_policy() now backfills the defaults when the key is
missing entirely, while still respecting a list the board intentionally
emptied and saved.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_reminders.py

@@ -1,3 +1,4 @@
+import json
 from datetime import date, timedelta
 from decimal import Decimal
 
@@ -278,3 +279,25 @@ def test_reminder_policy_rejects_invalid_input(tmp_path) -> None:
             levels=[{"name": "Mahnung", "fee": "-1.00", "payment_deadline_days": 14}],
             standard_fee_items=[],
         )
+
+
+def test_reminder_policy_backfills_standard_fee_items_for_old_repositories(tmp_path) -> None:
+    repository = MemberRepository(tmp_path)
+    repository.initialize()
+    config_path = tmp_path / "repository.json"
+    raw = json.loads(config_path.read_text(encoding="utf-8"))
+    # Simulate a repository created before "standard_fee_items" existed at all.
+    del raw["reminder_policy"]["standard_fee_items"]
+    config_path.write_text(json.dumps(raw), encoding="utf-8")
+
+    policy = repository.get_reminder_policy()
+    assert policy["standard_fee_items"], "should fall back to built-in defaults"
+    assert any(item["description"] == "Rücklastschriftgebühr" for item in policy["standard_fee_items"])
+
+    # But once the board explicitly saves an empty list, that choice is respected.
+    repository.save_reminder_policy(
+        grace_days_after_due=policy["grace_days_after_due"],
+        levels=policy["levels"],
+        standard_fee_items=[],
+    )
+    assert repository.get_reminder_policy()["standard_fee_items"] == []