feat(costs): create an expense from a booking, fix editing total-only items

Replace the inline price + budget-category fields in the Transport and
Reservation booking modals with a "Create expense" flow: the modal saves the
booking, then opens the full Costs editor prefilled (name + category mapped from
the booking type) and linked to the reservation. A booking with a linked expense
shows it inline with edit / remove.

Also fix the Costs editor so an expense with a recorded total but no payers
(transport-derived or pre-rework items) opens with its amount, lets you set the
currency, and saves - it previously showed 0 everywhere and could not be saved.
Legacy / localized categories now map to the fixed keys, and changing a booking's
type keeps its linked expense category in sync (unless it was manually set).

- shared: reservation_id on budget create, typeToCostCategory helper, i18n keys
- server: createBudgetItem stores reservation_id; keep total_price for payerless
  items; a booking update no longer wipes its linked expense and syncs the
  category on type change
- client: shared BookingCostsSection, exported ExpenseModal with prefill and an
  editable total, page-level save-then-open wiring

server/tests/integration/reservations.test.ts

@@ -382,7 +382,7 @@ describe('Reservation budget entry integration', () => {
     expect(items[0].total_price).toBe(150);
   });
 
-  it('RESV-014 — PUT without create_budget_entry removes existing linked budget item', async () => {
+  it('RESV-014 — PUT without create_budget_entry keeps the existing linked budget item', async () => {
     const { user } = createUser(testDb);
     const trip = createTrip(testDb, user.id);
 
@@ -398,24 +398,98 @@ describe('Reservation budget entry integration', () => {
     expect(createRes.status).toBe(201);
     const resvId = createRes.body.reservation.id;
 
-    // Verify budget item exists
     const before = testDb
       .prepare('SELECT id FROM budget_items WHERE trip_id = ? AND reservation_id = ?')
       .get(trip.id, resvId);
     expect(before).toBeDefined();
 
-    // Update without create_budget_entry — should delete the linked budget item
+    // Update WITHOUT create_budget_entry — the booking edit must NOT touch its
+    // linked expense (expenses are managed from the Costs section now).
     const updateRes = await request(app)
       .put(`/api/trips/${trip.id}/reservations/${resvId}`)
       .set('Cookie', authCookie(user.id))
       .send({ title: 'Taxi Updated' });
     expect(updateRes.status).toBe(200);
 
+    const after = testDb
+      .prepare('SELECT id FROM budget_items WHERE trip_id = ? AND reservation_id = ?')
+      .get(trip.id, resvId);
+    expect(after).toBeDefined();
+  });
+
+  it('RESV-014b — PUT with create_budget_entry total_price 0 removes the linked budget item', async () => {
+    const { user } = createUser(testDb);
+    const trip = createTrip(testDb, user.id);
+
+    const createRes = await request(app)
+      .post(`/api/trips/${trip.id}/reservations`)
+      .set('Cookie', authCookie(user.id))
+      .send({
+        title: 'Taxi',
+        type: 'transport',
+        create_budget_entry: { total_price: 50, category: 'Transport' },
+      });
+    expect(createRes.status).toBe(201);
+    const resvId = createRes.body.reservation.id;
+
+    // Explicit clear (total_price 0) still removes the linked item.
+    const updateRes = await request(app)
+      .put(`/api/trips/${trip.id}/reservations/${resvId}`)
+      .set('Cookie', authCookie(user.id))
+      .send({ title: 'Taxi', create_budget_entry: { total_price: 0 } });
+    expect(updateRes.status).toBe(200);
+
     const after = testDb
       .prepare('SELECT id FROM budget_items WHERE trip_id = ? AND reservation_id = ?')
       .get(trip.id, resvId);
     expect(after).toBeUndefined();
   });
+
+  it('RESV-014c — changing the booking type updates the linked expense category', async () => {
+    const { user } = createUser(testDb);
+    const trip = createTrip(testDb, user.id);
+
+    const createRes = await request(app)
+      .post(`/api/trips/${trip.id}/reservations`)
+      .set('Cookie', authCookie(user.id))
+      .send({ title: 'Booking', type: 'other', create_budget_entry: { total_price: 50, category: 'other' } });
+    const resvId = createRes.body.reservation.id;
+
+    // Change the type other -> hotel (no create_budget_entry).
+    await request(app)
+      .put(`/api/trips/${trip.id}/reservations/${resvId}`)
+      .set('Cookie', authCookie(user.id))
+      .send({ title: 'Booking', type: 'hotel' });
+
+    const item = testDb
+      .prepare('SELECT category FROM budget_items WHERE trip_id = ? AND reservation_id = ?')
+      .get(trip.id, resvId) as { category: string };
+    expect(item.category).toBe('accommodation');
+  });
+
+  it('RESV-014d — a manually-picked expense category survives a booking type change', async () => {
+    const { user } = createUser(testDb);
+    const trip = createTrip(testDb, user.id);
+
+    const createRes = await request(app)
+      .post(`/api/trips/${trip.id}/reservations`)
+      .set('Cookie', authCookie(user.id))
+      .send({ title: 'Booking', type: 'other', create_budget_entry: { total_price: 50, category: 'other' } });
+    const resvId = createRes.body.reservation.id;
+
+    // Simulate a manual category pick in the Costs editor.
+    testDb.prepare('UPDATE budget_items SET category = ? WHERE trip_id = ? AND reservation_id = ?').run('fees', trip.id, resvId);
+
+    await request(app)
+      .put(`/api/trips/${trip.id}/reservations/${resvId}`)
+      .set('Cookie', authCookie(user.id))
+      .send({ title: 'Booking', type: 'hotel' });
+
+    const item = testDb
+      .prepare('SELECT category FROM budget_items WHERE trip_id = ? AND reservation_id = ?')
+      .get(trip.id, resvId) as { category: string };
+    expect(item.category).toBe('fees');
+  });
 });
 
 describe('Reservation accommodation delete', () => {