Implement ETH send and QR code receive

Send: stores mnemonic/private key with wallet data, derives
signing key from mnemonic + address index via ethers HDNodeWallet,
constructs transaction with parseEther, broadcasts via
sendTransaction, waits for confirmation, shows block number
and tx hash. ENS resolution in To field preserved.

Receive: QR code rendered to canvas via qrcode library (1.5.4).
Shows scannable QR above the full address text.

README updated with qrcode dependency and TODO progress.

src/popup/index.html

@@ -288,6 +288,9 @@
                     Share this address with the sender. Make sure you only use
                     this address to receive Ethereum tokens.
                 </p>
+                <div class="flex justify-center mb-3">
+                    <canvas id="receive-qr"></canvas>
+                </div>
                 <div
                     id="receive-address"
                     class="border border-border p-2 break-all select-all mb-3"