Allow deleting claims, creating standalone payments, and add a donations tab

Claims could previously only be cancelled (stornieren), which blocks once a
payment is allocated. Add a hard delete that releases any linked
payments/credits back to being unallocated instead of destroying them.

Payments could only be created from within a claim, forcing immediate
allocation. Add a bare payment creation flow in the Zahlungen tab so
incoming transfers can be logged first and allocated later.

Add a per-member Spenden tab (donations, backed by a new donations list on
ContributionData) so amounts paid beyond the membership fee can be tracked
and existing free payments allocated to them.

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

src/ccma/ui/claim_tab.py

@@ -92,7 +92,9 @@ class ClaimTab(ttk.Frame):
         self.edit_button = ttk.Button(footer, text="Forderung bearbeiten", command=self._edit_claim)
         self.edit_button.grid(row=0, column=1, sticky="e", padx=(0, 8))
         self.cancel_button = ttk.Button(footer, text="Forderung stornieren", command=self._cancel_claim)
-        self.cancel_button.grid(row=0, column=2, sticky="e")
+        self.cancel_button.grid(row=0, column=2, sticky="e", padx=(0, 8))
+        self.delete_button = ttk.Button(footer, text="Forderung löschen", command=self._delete_claim)
+        self.delete_button.grid(row=0, column=3, sticky="e")
 
     def _build_ledger(self) -> None:
         ledger = ttk.Frame(self, padding=12)
@@ -503,6 +505,24 @@ class ClaimTab(ttk.Frame):
             return
         self._changed()
 
+    def _delete_claim(self) -> None:
+        allocated = allocated_total(self.data, self.claim_id)
+        detail = "Diese Forderung wirklich endgültig löschen? Das kann nicht rückgängig gemacht werden."
+        if allocated:
+            detail += (
+                f"\n\nZugeordnete Zahlungen/Gutschriften in Höhe von {money_text(allocated)} EUR "
+                "werden dabei gelöst und stehen danach wieder frei zur Verfügung."
+            )
+        if not messagebox.askyesno("Forderung löschen", detail, parent=self):
+            return
+        try:
+            self.repository.delete_claim(self.member_id, self.claim_id)
+        except RepositoryError as exc:
+            messagebox.showerror("Löschen fehlgeschlagen", str(exc), parent=self)
+            return
+        self.on_changed()
+        self.on_close()
+
     def _edit_claim(self) -> None:
         ClaimEditDialog(
             self,