Build your own group expense splitter (a personal Splitwise)

Build a link-shared expense splitter scaffold. Requirements:
- Node 22 with Express, better-sqlite3, server-rendered EJS templates, htmx copied into public/vendor, no SPA framework and no build step
- SQLite file at ./data/app.db with the path read from DATABASE_PATH in .env, commit a .env.example and gitignore .env
- Create tables: groups (id TEXT primary key, name TEXT, base_currency TEXT, created_at TEXT) and members (id INTEGER primary key, group_id TEXT, name TEXT)
- POST /groups creates a group whose id comes from nanoid at 21 characters, then redirects to /groups/<id>
- GET /groups/<id> lets anyone add members as plain typed names, no signup screens of any kind
- README states plainly that the unguessable link is the entire auth model
- Pain warning: better-sqlite3 compiles native code, so if npm install fails check for a C compiler and python3 before suspecting the code
- Out of scope: expense forms, balance math, styling, deployment

Add expense logging with flexible splits to the splitter from step 1. Requirements:
- Each expense stores payer member id, amount in integer cents, currency code, ISO date, note and category
- Support four split modes: equal, exact amounts, percentages and shares, validating that exact amounts sum to the total and percentages sum to 100 within a one cent tolerance
- Persist resolved results in an expense_splits table (expense_id, member_id, value_cents) so later steps read final numbers rather than raw mode inputs
- An htmx form posts to POST /groups/<id>/expenses and swaps in an updated expense list fragment, no full page reload
- Every edit and delete appends a row to an activity_log table (group_id, entity, entity_id, action, created_at)
- Log repayments as expenses with kind set to reimbursement so they can net against balances later
- Pain warning: float money will bite, keep every amount and split as integer cents including form parsing
- Out of scope: receipt photos, per-item splits, recurring expenses

Add the balances view and the settle-up plan to the existing app. Requirements:
- GET /groups/<id>/balances shows each member's net position: total paid minus total owed across expense_splits, with reimbursements netted in
- Write a simplifyDebts function implementing minimum cash flow debt simplification, repeatedly pairing the largest creditor with the largest debtor until every net reaches zero
- Render the result as a plain transfer list, one row per payment showing payer, recipient and amount, no buttons
- Add a unit test where A owes B, B owes C and C owes A, asserting the plan collapses to zero transfers
- Cover edge cases: a single-member group, and an equal split whose one cent remainder lands on the payer
- Pain warning: the greedy approach gives few transfers in practice but is not provably optimal, resist rewriting it as a graph flow problem
- Out of scope: interest calculations, date-aware settlement, any payment processing

Add multi-currency handling and exports to the group splitter. Requirements:
- Keep every expense amount in its original currency and add a rates table keyed by group and currency storing hand-entered rates to the group's base currency
- Build a small settings section on the group page where members edit the rate table, with no external exchange rate API calls
- Convert to the base currency only during balance computation, rounding once per member there
- On the expense list show original amount and converted base amount side by side
- Add GET /groups/<id>/export.csv and GET /groups/<id>/export.json streaming members, expenses, expense_splits and rates
- Fix the CSV column order and document it in the README so future imports stay predictable
- Pain warning: converting per display instead of per computation makes pennies drift between views, convert exactly once
- Out of scope: live rates, historical rate lookups, automatic currency detection