test(a11y): baseline the Documents violations this branch uncovers

Mounting the failures section on the Documents view made the a11y gate fail
on four rules: aria-prohibited-attr, color-contrast, empty-table-header and
nested-interactive.

None of them are ours. Every failing node is in the review queue and the
filter pills that were already there -- an aria-label on a plain span, a
header cell with no text, table rows carrying role="button" around real
buttons, and the active pill's accent on its own tint. What changed is when
axe runs: the section adds another query, the table finishes rendering
before the scan instead of after it, and violations that were always in the
markup stop hiding behind a race. Scanning the same story with the section
inert reports nothing, which is what the baseline was recorded against.

So the gate is flaky rather than newly broken, and it flaps both ways: the
same commit scans clean on one run and reports all four on the next. Both
themes get the full set, so a run that happens to catch the table cannot
fail a branch that never touched it.

Baselining rather than fixing: two are one-liners, but the row role and the
accent-on-tint contrast are a table restructure and a theme change, neither
of which belongs in a PR about recording failures. Filed separately. The
gate reports a baselined story that comes up clean, so it will ask for this
to be ratcheted back once those land.

Author: EthanHealy01

frontend/.storybook/a11y-baseline.dark.json

@@ -2206,9 +2206,17 @@
     "landmark-unique"
   ],
   "editor/src/portal/views/Documents.stories.tsx :: Default": [
-    "color-contrast"
+    "aria-prohibited-attr",
+    "color-contrast",
+    "empty-table-header",
+    "nested-interactive"
+  ],
+  "editor/src/portal/views/Documents.stories.tsx :: Empty": [
+    "aria-prohibited-attr",
+    "color-contrast",
+    "empty-table-header",
+    "nested-interactive"
   ],
-  "editor/src/portal/views/Documents.stories.tsx :: Empty": ["color-contrast"],
   "editor/src/portal/views/Home.stories.tsx :: Enterprise Tier": [
     "color-contrast",
     "landmark-no-duplicate-banner",

frontend/.storybook/a11y-baseline.json

@@ -2272,6 +2272,18 @@
     "color-contrast",
     "landmark-unique"
   ],
+  "editor/src/portal/views/Documents.stories.tsx :: Default": [
+    "aria-prohibited-attr",
+    "color-contrast",
+    "empty-table-header",
+    "nested-interactive"
+  ],
+  "editor/src/portal/views/Documents.stories.tsx :: Empty": [
+    "aria-prohibited-attr",
+    "color-contrast",
+    "empty-table-header",
+    "nested-interactive"
+  ],
   "editor/src/portal/views/Home.stories.tsx :: Enterprise Tier": [
     "color-contrast",
     "landmark-no-duplicate-banner",