feat: add weekly financial digest endpoint and UI

[bhubadinn]

Summary

• What changed: Added GET /digest/weekly?week=YYYY-WNN endpoint and a React Digest page with week navigation, category breakdown, WoW comparison, trends, and insights.
• Why: Closes Smart digest with weekly financial summary #121. Users need a weekly summary highlighting spending trends and patterns.

/claim #121

Validation

• Frontend lint: cd app && npm run lint
• Frontend tests: cd app && npm test -- --runInBand
• Backend tests: ./scripts/test-backend.ps1
• Updated docs if needed

Security and Ownership

• PR opened from a fork (not direct push to main)
• CODEOWNERS review requested

Checklist

• No secrets added
• No unrelated files changed
• Breaking changes documented

[bhubadinn]

Demo

Weekly Digest — W12 (2026-03-16 → 2026-03-22)

Features shown:

• GET /digest/weekly?week=2026-W12 returns total spent, total income, category breakdown with % shares, WoW change, trends and insights
• Week navigation (← →) switches between weeks and fetches fresh data
• Category breakdown with progress bars (Food & Dining 30.5%, Shopping 26%, Utilities 19.4%, Transportation 15.3%, Entertainment 8.8%)
• Week-over-week badge: +52.41% (red = spending increased)
• Trends: top category + WoW comparison text

W11 (2026-03-09 → 2026-03-15) — different data, different top category

• Shopping leads at 44%, Food & Dining 20.9%
• Confirms week isolation works correctly

API response sample

```json
GET /digest/weekly?week=2026-W12

{
"week": "2026-W12",
"period": { "start": "2026-03-16", "end": "2026-03-22" },
"total_spent": 691.93,
"total_income": 3200.00,
"week_over_week_change": 52.41,
"category_breakdown": [
{ "category_name": "Food & Dining", "amount": 211.25, "share_pct": 30.53 },
{ "category_name": "Shopping", "amount": 180.19, "share_pct": 26.04 },
{ "category_name": "Utilities", "amount": 134.00, "share_pct": 19.37 },
{ "category_name": "Transportation","amount": 105.50, "share_pct": 15.25 },
{ "category_name": "Entertainment", "amount": 60.99, "share_pct": 8.81 }
],
"trends": [
"Top spending category: Food & Dining (30.53% of total)",
"Spending increased 52.41% compared to previous week"
],
"insights": [
"You spent more than you earned this week",
"Spending spread across many categories this week"
]
}
```