fix: handle empty capacities array to prevent app crash

[moltboie]

Summary

Fixes the TypeError: Cannot read properties of undefined (reading 'month') crash that occurs when a budget/section/category has an empty capacities array.

Root Cause

Two budgets in the DB have capacities: []. When getActiveCapacity() is called:

1. sorted.find(...) returns undefined (empty array)
2. Fallback sorted[sorted.length - 1] evaluates to sorted[-1] → undefined
3. capacity[interval] (e.g., capacity['month']) throws on undefined

This crashes the entire app — the error boundary catches it but there's no recovery.

Fix

Client (BudgetFamily.ts):

• getActiveCapacity() now returns a default new Capacity() (month=0) when capacities is empty

Server (budgets.ts):

• createBudget, createSection, and createCategory now provide a default capacity ({month: 0, capacity_id: ...}) when none is specified, preventing empty arrays from being stored

Testing

1. Started app with existing empty-capacities budgets in DB
2. Verified app loads without crash — budgets page shows all budgets including the previously-crashing ones
3. Verified Dashboard and Accounts pages load correctly
4. TypeScript compiles clean (tsc --noEmit)

Closes #211