Stub shiki to eliminate 9MB of unused language grammars

vue-stream-markdown bundles all 359 shiki grammar chunks by default.
We only need plain text code blocks in chat. Alias shiki to an empty
module — build drops from 20MB/377 files to 11MB/103 files.

src/shims/shiki.ts

@@ -0,0 +1 @@
+throw new Error('shiki is stubbed out to reduce bundle size')

vite.config.ts

@@ -14,7 +14,8 @@ const host = process.env.TAURI_DEV_HOST
 export default defineConfig(async () => ({
   resolve: {
     alias: {
-      '@': resolve(__dirname, 'src')
+      '@': resolve(__dirname, 'src'),
+      shiki: resolve(__dirname, 'src/shims/shiki.ts')
     }
   },
   plugins: [