fix(submit): raise TinyMCE fullscreen above the fixed navbar

TinyMCE's skin injects .mce-fullscreen { z-index: 100 } at editor init,
after core editor.css, so the fullscreen editor (toolbar at the viewport
top) lost to the fixed navbar (.nav-fixed .header, z-index 999). Restore
core's intended 100010 with an element+class selector that outranks the
skin regardless of injection order; it stays below the submit media
dialog (100080). Also remove the dead .mce-top-part.affix rules (no
AKSRT script drives them) and pin both in a contract test.

.gitignore

@@ -56,6 +56,7 @@
 !/tests/front-submit/
 /tests/front-submit/*
 !/tests/front-submit/video-cover-contract.test.cjs
+!/tests/front-submit/editor-fullscreen-contract.test.cjs
 !/tests/asset-ledger/
 !/tests/asset-ledger/*.php
 !/tests/visual/docker-compose.zibll.yml