Fix dev mode to work without pre-built server binary

Previously, running `bun run dev` would fail because Tauri requires
the sidecar binary to exist at compile time, even in development mode.
This forced developers to build the full PyInstaller binary before
they could start development.

This change introduces a streamlined dev workflow:

1. Add `scripts/setup-dev-sidecar.js` - Creates minimal placeholder
   binaries that satisfy Tauri's compile-time check. Works cross-platform
   (Windows PE stub, Unix shell script).

2. Update Rust code to gracefully handle dev mode - When the sidecar
   fails to start, it checks if a manually-started server is already
   running on the expected port and connects to it instead.

3. Update npm scripts - `bun run dev` now auto-runs the setup script,
   and `dev:server` uses the correct port (17493).

4. Update CONTRIBUTING.md with clearer dev workflow documentation.

New development workflow:
  Terminal 1: bun run dev:server
  Terminal 2: bun run dev

The bundled binary is only required for production builds.

CONTRIBUTING.md

@@ -74,25 +74,32 @@ Thank you for your interest in contributing to Voicebox! This document provides
    This creates the SQLite database at `data/voicebox.db`.
 
 5. **Start development servers**
-   
-   **Terminal 1: Backend server**
+
+   Development requires two terminals: one for the Python backend, one for the Tauri app.
+
+   **Terminal 1: Backend server** (start this first)
    ```bash
    cd backend
    source venv/bin/activate  # Activate venv if not already active
    bun run dev:server
-   # Or manually: uvicorn main:app --reload --port 8000
+   # Or manually: uvicorn main:app --reload --port 17493
    ```
-   Backend will be available at `http://localhost:8000`
-   
+   Backend will be available at `http://localhost:17493`
+
    **Terminal 2: Desktop app**
    ```bash
    bun run dev
    ```
    This will:
+   - Create a placeholder sidecar binary (for Tauri compilation)
    - Start Vite dev server on port 5173
    - Launch Tauri window pointing to localhost:5173
+   - Connect to the Python server you started in Terminal 1
    - Enable hot reload
 
+   > **Note:** In dev mode, the app connects to your manually-started Python server.
+   > The bundled server binary is only used in production builds.
+
    **Optional: Web app**
    ```bash
    bun run dev:web