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>

build.mjs

@@ -0,0 +1,25 @@
+import * as esbuild from "esbuild";
+
+const watch = process.argv.includes("--watch");
+
+const options = {
+  entryPoints: [
+    "src/background.ts",
+    "src/content.ts",
+    "src/sciezka.ts",
+  ],
+  bundle: true,
+  outdir: "dist",
+  format: "iife",
+  target: "es2020",
+  sourcemap: watch,
+};
+
+if (watch) {
+  const ctx = await esbuild.context(options);
+  await ctx.watch();
+  console.log("Watching for changes...");
+} else {
+  await esbuild.build(options);
+  console.log("Build complete.");
+}