fix(ui): route tasks.js + skills.js dropdowns through topPortalZ() (#4768)

Fixes #4767. #4724 routed 16 body-portaled dropdowns through the shared
topPortalZ() helper so they always render just above the currently-raised tool
modal, but two were missed and still used a hardcoded z-index, so they hit the
same #4720 bug once a modal's bring-to-front counter climbed past the literal:

  - tasks.js _showTaskDropdown(): inline z-index:100000 on .task-dropdown
  - skills.js kebab menu (.skill-kebab-menu): z-index:100002 in style.css

Both now set zIndex from topPortalZ() after they are appended to the body,
matching the other migrated sites. The dead CSS z-index on .skill-kebab-menu is
removed (the inline value always wins). test_portal_dropdown_z_js.py gains a
source guard asserting both files use topPortalZ() and that no hardcoded
100000/100002 portal literal survives in either file or style.css.

static/style.css

@@ -16960,7 +16960,8 @@ body:not(.email-doc-split-active) #email-lib-modal.email-lib-fullscreen:not(.mod
 /* Kebab dropdown */
 .skill-kebab-menu {
   position: fixed;
-  z-index: 100002;
+  /* z-index is set inline via topPortalZ() at open time (#4720); a static
+     value here loses once the modal bring-to-front counter climbs past it. */
   min-width: 150px;
   padding: 4px;
   background: var(--panel, var(--bg));