Scaffold resource pack installer entry

Add a second Electron entry under src/installer-rp/ + installer-rp/
launched by `npm run installer:rp`. It is structurally separate from
the music quiz installer (own tsconfig, own preload, own renderer),
shares the existing styles via a relative link, and exposes a
three-step UI: pick a 음악퀴즈, run the install, then a 완료 page that
can open the resourcepacks folder or quit.

The install IPC handler currently scaffolds the flow per docs/add.md
(yt-dlp prep → music download → image normalize → zip build → place
at %appdata%/.minecraft/resourcepacks/) but the actual work is still
TODO. Cancel/cleanup of %appdata%/.mc_custom/.temp/ is wired up so
that future iterations can plug each step in without rewiring.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tsconfig.installer-rp.json

@@ -0,0 +1,4 @@
+{
+  "extends": "./tsconfig.json",
+  "include": ["src/installer-rp/**/*.ts", "src/shared/**/*.ts"]
+}