feat(import): add Splitwise API import for full expense history

[Mufaddal5253110]

Description

Adds direct Splitwise API import for complete expense history with individual transactions.

What it does:

• Imports individual expenses (not just net balances) with correct paid_share / owed_share calculations
• Preserves original createdAt timestamps for proper Activity ordering
• Uses transactionId to prevent duplicates (safe to re-run)

Files added:

• src/lib/splitwise-api.ts - API client with pagination
• src/lib/splitwise-converter.ts - Conversion logic
• src/server/api/routers/splitwiseApiImport.ts - tRPC endpoints
• src/pages/import-splitwise-api.tsx - UI for API key entry and group selection
• src/tests/splitwise-converter.test.ts - 29 test cases

Closes #307
Related to #465 (different approach - direct API vs external tool)

⚠️ ToS Discussion Needed

As noted in #307, Splitwise's API ToS prohibits use for competing services. This PR uses the direct API because the JSON export from export-splitwise only contains net balances, not individual expenses.

Options to discuss:

1. Keep API import with disclaimer (user provides their own key)
2. Remove and rely on CSV/JSON import only
3. Other suggestions?

Demo

1. User enters Splitwise API key (from secure.splitwise.com/apps)
2. Selects groups to import
3. Expenses imported with full history

Checklist

• I have read CONTRIBUTING.md in its entirety
• I have performed a self-review of my own code
• I have added unit tests to cover my changes
• The last commit successfully passed pre-commit checks
• Any AI code was thoroughly reviewed by me

[krokosik]

Sorry, I don't accept 100% vibe coded PRs and this is clearly one of them. Most importantly I cannot accept it due to the fact that it violates the Splitwise API ToS.