Embed frontend dist into Go binary; remove frontend path from health

- New backend/embed package embeds frontend dist/ via //go:embed
- Go binary serves frontend from embedded FS (no external files needed)
- Dev mode uses placeholder dir (dev.txt) so Go compiles; Vite handles
  frontend in dev mode
- Makefile copies frontend/dist/ to backend/embed/dist/ during build
- Removed FRONTEND_DIR env var and filesystem serving fallback
- Removed frontend path from Health API response and Status page frontend
  section (only Served boolean remains)
- Removed frontendDir from Handler struct/constructor (unused)

frontend/src/pages/Status.tsx

@@ -78,7 +78,6 @@ export default function Status() {
 
         <Section title="Frontend">
           <StatusRow label="Served" value={data.frontend.served ? 'Yes' : 'No'} ok={data.frontend.served} />
-          <StatusRow label="Path" value={data.frontend.path ? <CopyPath path={data.frontend.path} /> : '-'} />
         </Section>
       </div>
 

frontend/src/types/index.ts

@@ -77,7 +77,6 @@ export interface ServerHealth {
   }
   frontend: {
     served: boolean
-    path: string
   }
 }