chore(agent-office): convert agent PNGs to WebP (~93% smaller)

ffmpeg libwebp quality=85 compression_level=6.
Total: 11.8MB → 875KB (~11MB saved). Visually indistinguishable on
the card grid at the 9:16 image aspect.

PNG removals were already staged in the previous CommandTab commit;
this commit adds the 6 .webp replacements and points constants.js
imports at .webp.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/pages/agent-office/constants.js

@@ -1,10 +1,10 @@
 // src/pages/agent-office/constants.js
-import stockImg from './assets/agent_stock.png';
+import stockImg from './assets/agent_stock.webp';
-import musicImg from './assets/agent_music.png';
+import musicImg from './assets/agent_music.webp';
-import instaImg from './assets/agent_insta.png';
+import instaImg from './assets/agent_insta.webp';
-import realestateImg from './assets/agent_realestate.png';
+import realestateImg from './assets/agent_realestate.webp';
-import lottoImg from './assets/agent_lotto.png';
+import lottoImg from './assets/agent_lotto.webp';
-import undeterminedImg from './assets/agent_undetermined.png';
+import undeterminedImg from './assets/agent_undetermined.webp';
 
 export const AGENT_META = {
   stock:      { displayName: '주식 트레이더',   color: '#4488cc', image: stockImg },