Prefill the allocation amount when clicking an unallocated claim/donation

Clicking a claim or donation row in the payment allocation table used to
just show "0.00" if nothing was allocated to it yet, requiring the board
to type the amount by hand every time. It now suggests whichever is
smaller: what that claim/donation still has open, or what's left unused
of the payment -- so the common case is click the row, then Zuordnung
setzen, done. Rows that already have an allocation still show their
current value instead, so editing one isn't overwritten by the
suggestion.

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

src/ccma/ui/payment_dialog.py

@@ -42,12 +42,14 @@ class _AllocationTable:
         claim_allocations: dict[str, str],
         donation_allocations: dict[str, str],
         on_change: Callable[[], None],
+        get_payment_amount: Callable[[], Decimal],
     ):
         self.repository = repository
         self.member_id = member_id
         self.claim_allocations = claim_allocations
         self.donation_allocations = donation_allocations
         self.on_change = on_change
+        self.get_payment_amount = get_payment_amount
         # Fixed snapshot of what this payment already covered when the dialog opened.
         # Capacities are computed against this baseline rather than the live, editable
         # dicts above -- otherwise "Maximal zuordenbar" would inflate every time the
@@ -182,7 +184,23 @@ class _AllocationTable:
             return
         kind, target_id = self._target(selected[0])
         allocations = self.claim_allocations if kind == "claim" else self.donation_allocations
-        self.amount_var.set(allocations.get(target_id, "0.00"))
+        current = allocations.get(target_id)
+        if current:
+            # Already allocated: show the existing amount so it can be reviewed/edited.
+            self.amount_var.set(current)
+            return
+        # Not yet allocated: suggest whichever is smaller -- what this claim/donation
+        # still needs, or what's left of the payment -- so the common case is just
+        # "click the row, then Zuordnung setzen".
+        capacity = (self.claim_capacity if kind == "claim" else self.donation_capacity)[target_id]
+        free = max(self._payment_amount() - self.total_allocated(), Decimal("0"))
+        self.amount_var.set(money_text(min(capacity, free)))
+
+    def _payment_amount(self) -> Decimal:
+        try:
+            return self.get_payment_amount()
+        except ValueError:
+            return Decimal("0")
 
     def _set(self) -> None:
         selected = self.tree.selection()
@@ -314,6 +332,7 @@ class PaymentCreateDialog(tk.Toplevel):
             claim_allocations=self.claim_allocations,
             donation_allocations=self.donation_allocations,
             on_change=self._refresh_totals,
+            get_payment_amount=self._current_payment_amount,
         )
 
         ttk.Label(self, textvariable=self.total_var, style="Mono.TLabel").grid(
@@ -328,10 +347,13 @@ class PaymentCreateDialog(tk.Toplevel):
         ).pack(side="left")
         self._refresh_totals()
 
+    def _current_payment_amount(self) -> Decimal:
+        return decimal_value(self.variables["amount"].get())
+
     def _refresh_totals(self) -> None:
         allocated = self.allocation_table.total_allocated() if self.allocation_table else Decimal("0")
         try:
-            payment_amount = decimal_value(self.variables["amount"].get())
+            payment_amount = self._current_payment_amount()
             free = payment_amount - allocated
             self.total_var.set(f"Zugeordnet: {money_text(allocated)} EUR · Frei: {money_text(free)} EUR")
         except (ValueError, InvalidOperation):
@@ -448,6 +470,7 @@ class PaymentEditDialog(tk.Toplevel):
             claim_allocations=self.claim_allocations,
             donation_allocations=self.donation_allocations,
             on_change=self._refresh_totals,
+            get_payment_amount=self._current_payment_amount,
         )
 
         ttk.Label(self, textvariable=self.total_var, style="Mono.TLabel").grid(
@@ -474,10 +497,13 @@ class PaymentEditDialog(tk.Toplevel):
         except tk.TclError:
             return
 
+    def _current_payment_amount(self) -> Decimal:
+        return decimal_value(self.variables["amount"].get())
+
     def _refresh_totals(self) -> None:
         allocated = self.allocation_table.total_allocated() if self.allocation_table else Decimal("0")
         try:
-            payment_amount = decimal_value(self.variables["amount"].get())
+            payment_amount = self._current_payment_amount()
             free = payment_amount - allocated
             self.total_var.set(
                 f"Zugeordnet: {money_text(allocated)} EUR · Frei: {money_text(free)} EUR"