docs: add README

README.md

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+# Elden Ring Death Counter
+
+Elden Ring-themed death counter with real-time multi-client sync over WebSocket. Runs as a self-contained local server — no install required.
+
+## Pages
+
+| URL | Description |
+|---|---|
+| `http://localhost:8080` | Desktop page with YOU DIED overlay and keyboard controls |
+| `http://<ip>:8080/mobile` | Touch-optimized control page for phone |
+| `http://localhost:8080/obs` | Transparent browser source for OBS |
+
+## Run (dev)
+
+Requires [Deno](https://deno.com).
+
+```bash
+deno run --allow-net --allow-read --allow-write server.ts
+```
+
+Counter is persisted to `counter.json` in the working directory and survives restarts.
+
+## Build Windows .exe
+
+```bash
+deno compile \
+  --allow-net --allow-read --allow-write \
+  --include desktop.html \
+  --include mobile.html \
+  --include obs.html \
+  --target x86_64-pc-windows-msvc \
+  --output elden-counter.exe \
+  server.ts
+```
+
+Double-click `elden-counter.exe` — no runtime needed. Open `http://localhost:8080` in a browser.
+
+## Keyboard shortcuts (desktop)
+
+| Key | Action |
+|---|---|
+| `+` / `=` / `↑` | +1 |
+| `-` / `_` / `↓` | −1 |
+| `R` | Reset |