Implement personal_sign and eth_signTypedData_v4 message signing

Replace stub error handlers with full approval flow for personal_sign,
eth_sign, eth_signTypedData_v4, and eth_signTypedData. Uses toolbar
popup only (no fallback window) and keeps sign approvals pending across
popup close/reopen cycles so the user can respond via the toolbar icon.

src/popup/index.html

@@ -886,11 +886,7 @@
                 </div>
                 <div id="approve-tx-data-section" class="mb-3 hidden">
                     <div class="text-xs text-muted mb-1">Raw data</div>
-                    <div
-                        id="approve-tx-data"
-                        class="text-xs break-all"
-                        style="max-height: 4rem; overflow-y: auto"
-                    ></div>
+                    <div id="approve-tx-data" class="text-xs break-all"></div>
                 </div>
                 <div class="mb-2">
                     <label class="block mb-1 text-xs">Password</label>
@@ -917,6 +913,58 @@
                 </div>
             </div>
 
+            <!-- ============ SIGNATURE APPROVAL ============ -->
+            <div id="view-approve-sign" class="view hidden">
+                <h2 class="font-bold mb-2">Signature Request</h2>
+                <p class="mb-2">
+                    <span id="approve-sign-hostname" class="font-bold"></span>
+                    wants you to sign a message.
+                </p>
+
+                <div class="mb-3">
+                    <div class="text-xs text-muted mb-1">Type</div>
+                    <div id="approve-sign-type" class="text-xs font-bold"></div>
+                </div>
+
+                <div class="mb-3">
+                    <div class="text-xs text-muted mb-1">From</div>
+                    <div id="approve-sign-from" class="text-xs break-all"></div>
+                </div>
+
+                <div class="mb-3">
+                    <div class="text-xs text-muted mb-1">Message</div>
+                    <div
+                        id="approve-sign-message"
+                        class="text-xs break-all"
+                        style="max-height: 12rem; overflow-y: auto"
+                    ></div>
+                </div>
+
+                <div class="mb-2">
+                    <label class="block mb-1 text-xs">Password</label>
+                    <input
+                        type="password"
+                        id="approve-sign-password"
+                        class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
+                    />
+                </div>
+                <div id="approve-sign-error" class="text-xs hidden mb-2"></div>
+                <div class="flex justify-between">
+                    <button
+                        id="btn-approve-sign"
+                        class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
+                    >
+                        Sign
+                    </button>
+                    <button
+                        id="btn-reject-sign"
+                        class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
+                    >
+                        Reject
+                    </button>
+                </div>
+            </div>
+
             <!-- ============ SITE APPROVAL ============ -->
             <div id="view-approve-site" class="view hidden">
                 <h2 class="font-bold mb-2">Connection Request</h2>