fix: keep the dApp approval error containers to their reserved height (closes #297)

#approve-tx-error and #approve-sign-error reserved 20px, but their border
and padding took 10px of it, so a one-line error grew them to 26px and
pushed the Confirm/Sign buttons down 6px. They now reserve 30px, the same
4px to spare over one line that the other password error containers have.

A new end-to-end test shows each of the six password error containers on
its own screen at the popup viewport, empty and then with an error, and
fails if one changes height or the element below it moves.

Model: opus-5-5

src/popup/index.html

@@ -1633,7 +1633,7 @@
                 </div>
                 <div
                     id="approve-tx-error"
-                    class="text-xs mb-2 border border-border border-dashed p-1 min-h-[1.25rem]"
+                    class="text-xs mb-2 border border-border border-dashed p-1 min-h-[1.875rem]"
                     style="visibility: hidden"
                 ></div>
                 <div class="flex justify-between">
@@ -1710,7 +1710,7 @@
                 </div>
                 <div
                     id="approve-sign-error"
-                    class="text-xs mb-2 border border-border border-dashed p-1 min-h-[1.25rem]"
+                    class="text-xs mb-2 border border-border border-dashed p-1 min-h-[1.875rem]"
                     style="visibility: hidden"
                 ></div>
                 <div class="flex justify-between">