refactor: convert to bun workspaces monorepo

- Move web app into apps/web/
- Create packages/shared/ with shared types
- Create plugins/decky-vault/ scaffold
- Root package.json manages workspaces only

plugins/decky-vault/package.json

@@ -0,0 +1,19 @@
+{
+  "name": "@deckyvault/plugin",
+  "version": "0.1.0",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "build": "rollup -c",
+    "dev": "rollup -c -w"
+  },
+  "dependencies": {
+    "@deckyvault/shared": "workspace:*"
+  },
+  "devDependencies": {
+    "@rollup/plugin-commonjs": "^28",
+    "@rollup/plugin-typescript": "^12",
+    "rollup": "^4",
+    "typescript": "^5"
+  }
+}