feat(build): inject build time and version into health endpoint

Vite define injects __BUILD_TIME__ and __BUILD_VERSION__ at build time
so /health returns the exact timestamp and package version of the running
build — useful for verifying deploys without guessing what changed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app.d.ts

@@ -10,6 +10,9 @@ declare namespace App {
     // interface Platform {}
 }
 
+declare const __BUILD_TIME__: string;
+declare const __BUILD_VERSION__: string;
+
 declare global {
     namespace App {
         interface Platform {}