fix: use raw streaming chat completions and keep readable preview stacks

Switch OpenAI-compatible streaming from chat.completions.stream()
to chat.completions.create({ stream: true }) and track finish_reason
from streamed chunks directly.

This avoids the SDK's final chat-completion reconstruction path,
which could throw in preview builds while processing agent streams.

Also keep function/class names and sourcemaps in production builds
to make preview stack traces easier to read.

This issue can randomly happen when using `google/gemma-4-31b-it` model via OpenRouter.

Original error stacktrace:
```
index-Bh7OJFcp.js:807 Error processing stream: kn: Cannot read properties of undefined (reading 'type')
    at Bb.qJ (index-Bh7OJFcp.js:780:4436)Caused by: TypeError: Cannot read properties of undefined (reading 'type')
    at _J (index-Bh7OJFcp.js:780:1713)
    at index-Bh7OJFcp.js:780:416
    at Array.map (<anonymous>)
    at R6e (index-Bh7OJFcp.js:780:408)
    at z6e (index-Bh7OJFcp.js:785:258)
    at Bb.By (index-Bh7OJFcp.js:781:4928)
    at Bb._createChatCompletion (index-Bh7OJFcp.js:781:1059)
    at async Bb._runChatCompletion (index-Bh7OJFcp.js:780:6759)
```

vite.config.ts

@@ -16,6 +16,12 @@ export default defineConfig({
   define: {
     __APP_VERSION__: JSON.stringify(version),
   },
+  esbuild: {
+    keepNames: true,
+  },
+  build: {
+    sourcemap: true,
+  },
   server: {
     host: true,
     headers: {