Budget: per-person expense tracking with member chips

- New budget_item_members junction table (migration 27)
- Assign trip members to budget items via avatar chips in Persons column
- Per-person split auto-calculated from assigned member count
- Per-person summary integrated into total budget card
- Member chips rendered via portal dropdown (no overflow clipping)
- Mobile: larger touch-friendly chips (30px) under item name
- Desktop: compact chips (20px) in Persons column
- Custom NOMAD-style tooltips on chips
- WebSocket live sync for all member operations
- Fix invite button text color in dark mode
- Widen budget layout to 1800px max-width
- Shorten "Per Person/Day" column header

client/src/api/client.js

@@ -152,6 +152,9 @@ export const budgetApi = {
   create: (tripId, data) => apiClient.post(`/trips/${tripId}/budget`, data).then(r => r.data),
   update: (tripId, id, data) => apiClient.put(`/trips/${tripId}/budget/${id}`, data).then(r => r.data),
   delete: (tripId, id) => apiClient.delete(`/trips/${tripId}/budget/${id}`).then(r => r.data),
+  setMembers: (tripId, id, userIds) => apiClient.put(`/trips/${tripId}/budget/${id}/members`, { user_ids: userIds }).then(r => r.data),
+  togglePaid: (tripId, id, userId, paid) => apiClient.put(`/trips/${tripId}/budget/${id}/members/${userId}/paid`, { paid }).then(r => r.data),
+  perPersonSummary: (tripId) => apiClient.get(`/trips/${tripId}/budget/summary/per-person`).then(r => r.data),
 }
 
 export const filesApi = {