Update OKF docs bundle, README, and CLAUDE.md for today's features

Documents the shift planning and closure/payroll modules (new tables,
routes, stores), the dark redesign, and the docker-compose-broken-host
workaround. Adds CLAUDE.md pointing agents at docs/. Translates README to
English and keeps host-specific infrastructure details out of the repo.

docs/data-model/pay-rates.md

@@ -0,0 +1,47 @@
+---
+type: SQLite Table
+title: pay_rates
+description: Hourly rate history per employee, keyed by the date it takes effect.
+resource: backend/src/db/index.ts
+tags: [data-model, sqlite, payroll]
+timestamp: 2026-08-16T00:00:00Z
+---
+
+# pay_rates
+
+Versioned hourly rate per employee. There's no `valid_to` column — the
+active rate for a given date is simply the row with the largest
+`valid_from <= that date` (`getRateAt` in `backend/src/services/payRates.ts`).
+A new rate implicitly ends the previous one.
+
+# Schema
+
+| Column | Type | Description |
+|---|---|---|
+| `id` | INTEGER PK | autoincrement |
+| `employee_id` | INTEGER | references [employees](./employees.md) |
+| `hourly_rate` | REAL | Kč/hour |
+| `valid_from` | TEXT | `YYYY-MM-DD`, takes effect from this date inclusive |
+| `created_at` | TEXT | ISO 8601 UTC |
+
+`UNIQUE (employee_id, valid_from)` — setting a rate again for the same date
+overwrites it (`ON CONFLICT ... DO UPDATE`) rather than erroring.
+
+# Behavior
+
+- **First rate ever set for an employee** defaults `valid_from` to the
+  employee's own `created_at` date when the admin doesn't specify one — so
+  hours already worked before the admin got around to entering a rate
+  still get paid correctly, rather than pricing at 0. This was a real bug
+  found live: an admin set a rate today, and it didn't apply to shifts
+  worked earlier that same month.
+- **Every later rate change** (a raise) defaults to today instead — not
+  retroactive. This asymmetry lives in `defaultValidFrom` in
+  `backend/src/services/payRates.ts` and is covered by a test in
+  `backend/src/services/payroll.test.ts`.
+
+# Related
+
+- [employees](./employees.md)
+- [month_closures](./month-closures.md) / [payroll](./payroll.md) — consume
+  `getRateAt` when computing `earned_estimate` / `base_amount`