feat: add dynamic budget suggestions endpoint

[Mohye24k]

Summary

• Add GET /budget-suggest endpoint based on 90-day spending history
• Suggest per-category budgets using the 50/30/20 rule (needs/wants/savings)
• Return suggestions[], monthly_income, savings_target, and budget_rule breakdown
• Add comprehensive test suite (auth required, empty state, with data)
• Add TypeScript API client (app/src/api/budgetSuggest.ts)
• Register blueprint in __init__.py

Test plan

• Auth required returns 401
• Empty state returns zeroed suggestions
• With data returns category suggestions with 10% buffer
• Budget rule follows 50/30/20 allocation

/claim #73
Fixes #73