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/index.html

@@ -307,6 +307,16 @@
                     </button>
                 </div>
 
+                <!-- actions row 2 -->
+                <div class="flex gap-2 mb-3">
+                    <button
+                        id="btn-export-privkey"
+                        class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer flex-1 text-xs"
+                    >
+                        Export Private Key
+                    </button>
+                </div>
+
                 <!-- transactions -->
                 <div class="mt-3">
                     <div class="border-b border-border pb-1 mb-1">
@@ -318,6 +328,49 @@
                 </div>
             </div>
 
+            <!-- ============ EXPORT PRIVATE KEY VIEW ============ -->
+            <div id="view-export-privkey" class="view hidden">
+                <button
+                    id="btn-export-privkey-back"
+                    class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer mb-2"
+                >
+                    &lt; Back
+                </button>
+                <h2 class="font-bold mb-3">Export Private Key</h2>
+                <p class="text-xs mb-1" id="export-privkey-label"></p>
+                <p class="text-xs mb-3 text-muted">
+                    Warning: anyone with this private key can access and
+                    transfer all funds from this address. Never share it.
+                </p>
+                <div
+                    id="export-privkey-flash"
+                    class="text-xs mb-2 hidden"
+                ></div>
+                <div id="export-privkey-password-section" class="mb-2">
+                    <label class="block mb-1">Password</label>
+                    <input
+                        type="password"
+                        id="export-privkey-password"
+                        class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
+                        placeholder="Enter your password to continue"
+                    />
+                    <button
+                        id="btn-export-privkey-confirm"
+                        class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer mt-2"
+                    >
+                        Reveal
+                    </button>
+                </div>
+                <div id="export-privkey-result" class="hidden">
+                    <div
+                        id="export-privkey-value"
+                        class="border border-dashed border-border p-2 font-mono text-xs break-all cursor-pointer mb-1"
+                        title="Click to copy"
+                    ></div>
+                    <p class="text-xs text-muted">Click to copy.</p>
+                </div>
+            </div>
+
             <!-- ============ ADDRESS-TOKEN DETAIL VIEW ============ -->
             <div id="view-address-token" class="view hidden">
                 <button

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 };