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.

tests/front-submit/editor-fullscreen-contract.test.cjs

@@ -0,0 +1,35 @@
+'use strict';
+
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+const test = require('node:test');
+
+const root = path.resolve(__dirname, '..', '..');
+const read = (file) => fs.readFileSync(path.join(root, file), 'utf8');
+
+test('submit.css raises the TinyMCE fullscreen container above the fixed navbar', () => {
+  const css = read('assets/css/frontend/pages/submit.css');
+
+  // TinyMCE's skin injects .mce-fullscreen { z-index: 100 } at editor init,
+  // after core editor.css, so the fullscreen editor loses to the fixed navbar
+  // (.nav-fixed .header, z-index 999). The element+class selector outranks the
+  // skin regardless of injection order and restores core's intended 100010,
+  // which stays below the submit media dialog (100080).
+  assert.match(css, /div\.mce-fullscreen \{\s*\n  z-index: 100010;\s*\n\}/);
+});
+
+test('the Zibll toolbar affix rules stay removed', () => {
+  const css = read('assets/css/frontend/pages/submit.css');
+  const jsRoot = path.join(root, 'assets/js');
+  const js = fs
+    .readdirSync(jsRoot, { withFileTypes: true })
+    .filter((entry) => entry.isFile())
+    .map((entry) => read(`assets/js/${entry.name}`))
+    .join('\n');
+
+  // No AKSRT script adds .affix to .mce-top-part (that was Zibll's
+  // editextend.js), so the affix positioning rules must not come back.
+  assert.doesNotMatch(css, /mce-top-part\.affix/);
+  assert.doesNotMatch(js, /mce-top-part/);
+});