feat: add export private key from address detail view

Adds an 'Export Private Key' button to the address detail view.
Clicking it opens a password confirmation screen; after verification,
the derived private key is displayed in a copyable field with a
security warning. The key is cleared when navigating away.

Closes #19

src/popup/views/addressDetail.js

@@ -18,6 +18,8 @@ const { resolveEnsNames } = require("../../shared/ens");
 const { updateSendBalance, renderSendTokenSelect } = require("./send");
 const { log } = require("../../shared/log");
 const makeBlockie = require("ethereum-blockies-base64");
+const { decryptWithPassword } = require("../../shared/vault");
+const { getSignerForAddress } = require("../../shared/wallet");
 
 let ctx;
 
@@ -261,6 +263,64 @@ function init(_ctx) {
     });
 
     $("btn-add-token").addEventListener("click", ctx.showAddTokenView);
+
+    $("btn-export-privkey").addEventListener("click", () => {
+        const wallet = state.wallets[state.selectedWallet];
+        const addr = wallet.addresses[state.selectedAddress];
+        const title = addressTitle(addr.address, state.wallets);
+        const label = title || "Address " + (state.selectedAddress + 1);
+        $("export-privkey-label").textContent = "Exporting key for: " + label;
+        $("export-privkey-password").value = "";
+        $("export-privkey-flash").classList.add("hidden");
+        $("export-privkey-flash").textContent = "";
+        $("export-privkey-password-section").classList.remove("hidden");
+        $("export-privkey-result").classList.add("hidden");
+        $("export-privkey-value").textContent = "";
+        showView("export-privkey");
+    });
+
+    $("btn-export-privkey-confirm").addEventListener("click", async () => {
+        const password = $("export-privkey-password").value;
+        if (!password) {
+            $("export-privkey-flash").textContent = "Password is required.";
+            $("export-privkey-flash").classList.remove("hidden");
+            return;
+        }
+        const wallet = state.wallets[state.selectedWallet];
+        try {
+            const secret = await decryptWithPassword(
+                wallet.encryptedSecret,
+                password,
+            );
+            const signer = getSignerForAddress(
+                wallet,
+                state.selectedAddress,
+                secret,
+            );
+            const privateKey = signer.privateKey;
+            $("export-privkey-password-section").classList.add("hidden");
+            $("export-privkey-value").textContent = privateKey;
+            $("export-privkey-result").classList.remove("hidden");
+            $("export-privkey-flash").classList.add("hidden");
+        } catch {
+            $("export-privkey-flash").textContent = "Wrong password.";
+            $("export-privkey-flash").classList.remove("hidden");
+        }
+    });
+
+    $("export-privkey-value").addEventListener("click", () => {
+        const key = $("export-privkey-value").textContent;
+        if (key) {
+            navigator.clipboard.writeText(key);
+            showFlash("Copied!");
+        }
+    });
+
+    $("btn-export-privkey-back").addEventListener("click", () => {
+        $("export-privkey-value").textContent = "";
+        $("export-privkey-password").value = "";
+        show();
+    });
 }
 
 module.exports = { init, show };