fix: make export key a subtle link, add view to VIEWS array

- Moved 'Export private key' from prominent button row to a small
  muted text link at the bottom of the address detail view
- Added 'export-privkey' to the VIEWS array in helpers.js — this was
  the cause of the blank view (showView toggled all known views but
  didn't know about export-privkey, so it was never unhidden)

src/popup/index.html

@@ -307,16 +307,6 @@
                     </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">
@@ -326,6 +316,13 @@
                         <div class="text-muted text-xs py-1">Loading...</div>
                     </div>
                 </div>
+                <div class="mt-3 text-center">
+                    <span
+                        id="btn-export-privkey"
+                        class="text-xs text-muted underline decoration-dashed cursor-pointer hover:text-fg"
+                        >Export private key</span
+                    >
+                </div>
             </div>
 
             <!-- ============ EXPORT PRIVATE KEY VIEW ============ -->