Fix layout shift from ETH price well and Total subtitle

Both elements now have min-height and placeholder content so they
reserve vertical space before API data arrives, preventing layout
jumps when prices load. This follows the No Layout Shift policy
in the README.

src/popup/index.html

@@ -177,7 +177,12 @@
                     id="total-value"
                     class="text-2xl font-bold min-h-[2rem]"
                 ></div>
-                <div id="total-value-sub" class="text-xs text-muted mb-2"></div>
+                <div
+                    id="total-value-sub"
+                    class="text-xs text-muted mb-2 min-h-[1rem]"
+                >
+                    &nbsp;
+                </div>
 
                 <!-- active address display -->
                 <div
@@ -204,8 +209,10 @@
                 <!-- ETH price -->
                 <div
                     id="eth-price-display"
-                    class="bg-well text-center font-bold text-xs p-1 mx-1 mb-3"
-                ></div>
+                    class="bg-well text-center font-bold text-xs p-1 mx-1 mb-3 min-h-[1.5rem]"
+                >
+                    &nbsp;
+                </div>
 
                 <!-- wallet list -->
                 <div id="wallet-list"></div>