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.

manifest/chrome.json

@@ -16,5 +16,11 @@
             "js": ["src/content/index.js"],
             "run_at": "document_start"
         }
+    ],
+    "web_accessible_resources": [
+        {
+            "resources": ["src/content/inpage.js"],
+            "matches": ["<all_urls>"]
+        }
     ]
 }