Initial implementation: relay server + common protocol + client stub

crates/client/README.md

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+# helios-client (Phase 2 — not yet implemented)
+
+This crate will contain the Windows remote-control client for `helios-remote`.
+
+## Planned Features
+
+- Connects to the relay server via WebSocket (`wss://`)
+- Sends a `Hello` message on connect with an optional display label
+- Handles incoming `ServerMessage` commands:
+  - `ScreenshotRequest` → captures the primary display (Windows GDI or `windows-capture`) and responds with base64 PNG
+  - `ExecRequest` → runs a shell command in a persistent `cmd.exe` / PowerShell session and returns stdout/stderr/exit-code
+  - `ClickRequest` → simulates a mouse click via `SendInput` Win32 API
+  - `TypeRequest` → types text via `SendInput` (virtual key events)
+- Persistent shell session so `cd C:\Users` persists across `exec` calls
+- Auto-reconnect with exponential backoff
+- Configurable via environment variables or a `client.toml` config file
+
+## Planned Tech Stack
+
+| Crate | Purpose |
+|---|---|
+| `tokio` | Async runtime |
+| `tokio-tungstenite` | WebSocket client |
+| `serde_json` | Protocol serialization |
+| `windows` / `winapi` | Screen capture, mouse/keyboard input |
+| `base64` | PNG encoding for screenshots |
+
+## Build Target
+
+```
+cargo build --target x86_64-pc-windows-gnu
+```