feat: CUDA backend swap via binary download and restart

Add the ability to download a CUDA-enabled backend binary (~2.4 GB) and
swap it in via a backend-only restart, solving the #1 user pain point
(19 open 'GPU not detected' issues caused by GitHub's 2 GB asset limit).

Backend:
- cuda_download.py: download from R2 (primary) or GitHub split-parts
  (fallback), SHA-256 verification, atomic writes, progress via SSE
- 4 new endpoints: GET/POST/DELETE /backend/cuda-*, GET cuda-progress
- server.py: --version flag, auto-detect variant from binary name
- build_binary.py: --cuda flag for CUDA PyInstaller builds
- split_binary.py: split large binaries into <2GB GitHub Release assets
- CI workflow for building CUDA binary

Tauri:
- restart_server command (stop -> wait -> start)
- start_server prefers CUDA binary from {data_dir}/backends/ if present
- Version mismatch check: runs --version before launching CUDA binary

Frontend:
- GpuAcceleration component: download, progress, restart, switch, delete
- API client + types for CUDA status and management
- Platform lifecycle: restartServer() on Tauri/Web
- Aggressive 1s health polling during restart for fast reconnection

tauri/src/platform/lifecycle.ts

@@ -1,5 +1,5 @@
 import { invoke } from '@tauri-apps/api/core';
-import { listen, emit } from '@tauri-apps/api/event';
+import { emit, listen } from '@tauri-apps/api/event';
 import type { PlatformLifecycle } from '@/platform/types';
 
 class TauriLifecycle implements PlatformLifecycle {
@@ -27,6 +27,18 @@ class TauriLifecycle implements PlatformLifecycle {
     }
   }
 
+  async restartServer(): Promise<string> {
+    try {
+      const result = await invoke<string>('restart_server');
+      console.log('Server restarted:', result);
+      this.onServerReady?.();
+      return result;
+    } catch (error) {
+      console.error('Failed to restart server:', error);
+      throw error;
+    }
+  }
+
   async setKeepServerRunning(keepRunning: boolean): Promise<void> {
     try {
       await invoke('set_keep_server_running', { keepRunning });