fix(userlist): stop username input from overlapping the Log out button

Fixes #7593. In the pad's Users popup, #myusernameform had no width
set and the <input id="myusernameedit"> inside it took its natural
content width, pushing past the Log out button and making the button
overflow the popup at common widths.

Constrain #myusernameform to 75px and make the input fill its
container with box-sizing: border-box so the text field stays inside
the form and the Log out button sits visibly next to it rather than
getting covered or clipped off-screen.

Low-risk, CSS-only change. No test plan beyond visual verification
because the affected control is in the users popup UI.

Author: JohnMcLear

src/static/css/pad/popup_users.css

@@ -58,6 +58,10 @@
 
 #myusernameform {
   margin-left: 10px;
+  /* Constrain the username input so the adjacent Log out button does not
+     get pushed off the Users popup — previously the input expanded to its
+     natural default width and overlapped the button. Fixes #7593. */
+  width: 75px;
 }
 input#myusernameedit {
   height: 26px;
@@ -66,6 +70,11 @@ input#myusernameedit {
   border: 1px solid #ccc;
   background-color: transparent;
   margin: 0;
+  /* Keep the input inside its 75px container (border + padding included).
+     Without these the text field renders wider than the container on its
+     natural content width and overlaps the Log out button. */
+  width: 100%;
+  box-sizing: border-box;
 }
 input#myusernameedit:not(.editable) {
   color: grey;