## Summary
Budgets previously tracked a single category **or** label (mutually
exclusive). This lets a budget span **multiple** categories and labels
at once, all pooling spend against one allocated amount per period.
Scope decided with the requester:
- **Shared pool** — one allocated amount; any tracked category/label
counts against it.
- **Multi categories + multi labels** on a single budget.
- **Create-only** — tracking is chosen at creation and locked afterward
(edit dialog shows it read-only).
## Changes
**Backend**
- New `budget_category` + `budget_label` pivot tables; data migration
copies existing `category_id`/`label_id` into them, then drops those
columns.
- `Budget` model: `categories()` / `labels()` belongsToMany.
- `BudgetTransactionService` matches transactions across **all** tracked
categories OR labels (live assignment + historical backfill).
- `StoreBudgetRequest` accepts `category_ids` / `label_ids` arrays,
requires ≥1 across both, validates ownership. `update` no longer touches
tracking.
**Frontend**
- Reusable `MultiSelect` (popover + command + badge).
- Create dialog uses multi-selects; cards and show page render tracked
categories/labels as badges; edit dialog shows them read-only.
- Dexie bumped to v10 (drops unused per-category allocations table, adds
`budget_labels`).
## Testing
- Updated all budget/transaction/listener/browser tests for the pivot
model; added cases for multi-category matching, mixed category+label
pooling, and store validation (empty selection + foreign-ownership).
- Added the new `__()` strings to `lang/es.json`.
- Local `pint`, `lint`, `format` pass. Relying on CI for the full suite.
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