Implement EIP-1193 provider for dApp connectivity

Three-part architecture:
- inpage.js: creates window.ethereum in page context with
  request(), on(), send(), sendAsync(), enable() methods.
  Sets isMetaMask=true for compatibility.
- content/index.js: bridge between page and extension via
  postMessage (page<->content) and runtime.sendMessage
  (content<->background).
- background/index.js: handles RPC routing. Proxies read-only
  methods (eth_call, eth_getBalance, etc.) to configured RPC.
  Handles eth_requestAccounts (auto-connect for now),
  wallet_switchEthereumChain (mainnet only), and returns
  informative errors for unimplemented signing methods.

Manifests updated with web_accessible_resources for inpage.js.
Build updated to bundle inpage.js as a separate output file.

build.js

@@ -62,6 +62,17 @@ async function build() {
             minify: true,
         });
 
+        // bundle inpage script (injected into page context, separate file)
+        await esbuild.build({
+            entryPoints: [path.join(SRC, "content", "inpage.js")],
+            bundle: true,
+            format: "iife",
+            outfile: path.join(distDir, "src", "content", "inpage.js"),
+            platform: "browser",
+            target: ["chrome110", "firefox110"],
+            minify: true,
+        });
+
         // copy popup HTML
         fs.copyFileSync(
             path.join(SRC, "popup", "index.html"),