chore: add CLAUDE.md with architecture and development guidance

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Commands
+
+```bash
+npm install        # install dev dependencies (vitest only)
+npm test           # run all tests once
+npm run test:watch # run tests in watch mode
+
+# run a single test file
+npx vitest run src/elo.test.js
+```
+
+No build step. Open `index.html` directly in a browser or serve it statically.
+
+## Architecture
+
+The app is a no-bundler ES-module SPA. Modules load via `<script type="module">` in `index.html`; Chart.js comes from CDN ESM.
+
+**Layer order (strict dependency direction):**
+
+```
+elo.js → api.js → state.js → ui.js → app.js
+                    ↑
+         branding.js  streaks.js  chart.js  demo.js
+```
+
+- **`src/elo.js`** — pure ELO math, no side effects. K=32, start ELO=1000. Doubles use average team ELO.
+- **`src/api.js`** — all Supabase REST calls. Must call `initApi(url, key, secret)` first. Owns camelCase↔snake_case mapping. Appends `x-app-secret` header on every request when secret is set.
+- **`src/state.js`** — single mutable `state` object (`players`, `matches`, `selectedPlayers`, `currentGameMode`). `recalculateStatsFromHistory()` replays all matches chronologically to recompute every player stat — call this after any match add/delete to avoid drift. localStorage is the offline cache layer.
+- **`src/ui.js`** — pure DOM rendering. Takes callbacks for interactive elements (e.g. `renderRankings(onRowClick)`). Never calls API or mutates state directly.
+- **`src/chart.js`** — Chart.js 4 wrappers. `renderEloChart()` for global view, `renderPlayerChart()` for profile modal.
+- **`src/streaks.js`** — calculates current/longest win-loss streaks from `state.matches` for a given player.
+- **`src/branding.js`** — `applyBranding(branding)` sets CSS custom properties on `:root` and updates `<title>`, `<h1>`, meta tags. All brand values flow from `config.js`.
+- **`src/demo.js`** — hardcoded sample players and matches for `?demo=true` mode.
+- **`app.js`** — orchestration only. Wires up events, calls API, updates state, then calls render functions. Also exposes certain functions on `window.*` for HTML `onclick` attributes.
+
+## Configuration
+
+`config.js` is **gitignored** — never commit it. Copy `config.example.js` → `config.js` and fill in:
+- `SUPABASE_URL`, `SUPABASE_ANON_KEY` — from Supabase project settings
+- `APP_SECRET` — write-protection password, sent as `x-app-secret` header
+- `BRANDING` — optional white-label overrides (name, colors, fonts)
+
+`CONFIG` is declared with `var` (not `const`) so it's available in global scope before ES modules load.
+
+## Key Invariants
+
+- **ELO source of truth is the match history**, not the stored player ELO values. Always call `recalculateStatsFromHistory()` after loading or deleting matches.
+- **Doubles matches** store `winnerId`/`loserId` as comma-separated player ID strings (`"id1,id2"`). Check for commas before parsing.
+- **Write operations** require `APP_SECRET`; reads are public. Supabase RLS enforces this via the `x-app-secret` header.
+- The service worker (`sw.js`, cache key `speedhennen-v3`) uses cache-first for static assets and network-first for Supabase/CDN. Bump the cache version string when adding new static files to `STATIC_ASSETS`.