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.

src/content/index.js

@@ -1,2 +1,51 @@
-// AutistMask content script
-// TODO: inject window.ethereum provider into page context
+// AutistMask content script — bridges between inpage (window.ethereum)
+// and the background service worker via extension messaging.
+
+// Inject the inpage script into the page's JS context
+const script = document.createElement("script");
+script.src = (typeof browser !== "undefined" ? browser : chrome).runtime.getURL(
+    "src/content/inpage.js",
+);
+script.onload = function () {
+    this.remove();
+};
+(document.head || document.documentElement).appendChild(script);
+
+// Relay requests from the page to the background script
+window.addEventListener("message", (event) => {
+    if (event.source !== window) return;
+    if (event.data?.type !== "AUTISTMASK_REQUEST") return;
+    const { id, method, params } = event.data;
+
+    const runtime =
+        typeof browser !== "undefined" ? browser.runtime : chrome.runtime;
+
+    runtime.sendMessage(
+        { type: "AUTISTMASK_RPC", id, method, params, origin: location.origin },
+        (response) => {
+            if (response) {
+                window.postMessage(
+                    { type: "AUTISTMASK_RESPONSE", id, ...response },
+                    "*",
+                );
+            }
+        },
+    );
+});
+
+// Listen for events pushed from the background (e.g. accountsChanged)
+const runtime =
+    typeof browser !== "undefined" ? browser.runtime : chrome.runtime;
+
+runtime.onMessage.addListener((msg) => {
+    if (msg.type === "AUTISTMASK_EVENT") {
+        window.postMessage(
+            {
+                type: "AUTISTMASK_EVENT",
+                eventName: msg.eventName,
+                data: msg.data,
+            },
+            "*",
+        );
+    }
+});