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

packages/shared/package.json

@@ -0,0 +1,11 @@
+{
+  "name": "@deckyvault/shared",
+  "version": "0.1.0",
+  "private": true,
+  "type": "module",
+  "main": "./src/index.ts",
+  "types": "./src/index.ts",
+  "devDependencies": {
+    "typescript": "^5"
+  }
+}