feat: project infrastructure and MV3 extension setup

TypeScript + esbuild build system, Manifest V3 with background script,
content script for iframe overlay injection, and typed message contracts
between extension contexts.

Assisted-by: Claude Code
Signed-off-by: Avinal Kumar <avinal.xlvii@gmail.com>

tsconfig.json

@@ -0,0 +1,17 @@
+{
+  "compilerOptions": {
+    "target": "ES2020",
+    "module": "ESNext",
+    "moduleResolution": "bundler",
+    "strict": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "noFallthroughCasesInSwitch": true,
+    "isolatedModules": true,
+    "skipLibCheck": true,
+    "outDir": "dist",
+    "rootDir": "src",
+    "lib": ["ES2020", "DOM", "DOM.Iterable"]
+  },
+  "include": ["src/**/*.ts"]
+}