chore: drop orphan Phaser-era assets

bg.png (295 KB) and logo.png (24 KB) had zero references in src,
index.html, or any CSS, but matched the workbox precache glob and
shipped to every install. Removing them drops the precache from
555 KB to 235 KB (-58%). Update the docs note.

Also add the review-fixes plan to plans/.

docs/codebase-summary.md

@@ -25,7 +25,7 @@ src/
 public/
 ├── style.css                       # Legacy file — theme now lives in src/app.css
 ├── favicon.png
-└── assets/                         # bg.png, logo.png (unused, reserved)
+└── assets/                         # qr.jpg (donate VietQR)
 ```
 
 ## Data flow