fix(expenses): add fetch_categories tool and category field to add-expense form

[theprogrammersingh]

Problem

1. When calling submit_expense, its schema previously accepted a category property (name string like "Travel"), but the backend's CreateExpenseDto expects categoryId (UUID) with ValidationPipe({ whitelist: true, forbidNonWhitelisted: true }). This caused a 400 Bad Request error when category was passed, and recent entries lacked a category_id in the database.
2. The Add Expense form had no category selector.

Solution

1. fetch_categories Tool:
   • Added a new read-only WebMCP tool contract (FETCH_CATEGORIES) in @actuo/shared and implemented in ExpenseTools.
   • Returns the organization's categories (id, name, icon) so the model can resolve human-readable category names to valid categoryId UUIDs before filing.
   • Included in ALWAYS_ON_TOOLS.
2. submit_expense Tool Contract:
   • Updated input schema to take categoryId UUID, aligned directly with backend CreateExpenseDto.
   • Updated description instructing the agent to invoke fetch_categories first when a user mentions a category.
3. Add Expense Form:
   • Added a <select id="categoryId" name="categoryId"> dropdown populated asynchronously from GET /api/orgs/current/categories.
   • Annotated with declarative WebMCP attributes.
   • Forwards categoryId in the creation payload.
4. Tests:
   • Added unit tests for fetch_categories and categoryId submission in expense-tools.spec.ts.
   • Added tests for category dropdown rendering and submission in add-expense.spec.ts.
   • Updated tool list assertions in gemini-schema.spec.ts and tool-session.spec.ts.

Verification

• Frontend unit tests: 46/46 passed (821 tests)
• Backend unit tests: 10/10 passed (142 tests)
• Full monorepo build (pnpm build): passed