refactor(docs): remove two dead CSS rules

[mfal]

Deletes two apps/docs CSS rules that match nothing in the rendered DOM. No
rendering change — verified, see below.

Follow-up from the selector-max-type triage in #3091, where these three
warnings were disabled rather than changed, because switching them to a class
selector would have activated them.

1. .mainContent > h3, > h4 { margin-top } — obsolete

apps/docs/src/app/layout.module.scss

Live until #2573 wrapped the MDX output in <div style="display: contents">.
display: contents removes the box from layout but not from the DOM, so the
child combinator stopped matching and the extra spacing silently disappeared.

#2870 ("docs: fix heading spacing") fixed that at the heading instead:

.anchorLinkHeading {
  &h3 { margin-top: var(--section--sub-heading-spacing-m); }
}

That is what puts 24px above every MDX ## today. The layout rule has been
redundant since.

Note

#3091's comment on this rule has the reasoning backwards — "the child
combinator is load-bearing … only headings rendered directly here match".
Because the MDX view wraps its output, none match.

2. .mobileNavigationOffCanvas h3 { display: none } — never applied

apps/docs/src/app/_components/layout/Header/Header.module.scss

A styles import/class mismatch, dead since the day it was written. #2338 put
the class in Header.module.scss, but its only consumer,
MobileNavigation.tsx, imports @/app/layout.module.scss — which has no such
key. styles.mobileNavigationOffCanvas was undefined, so React dropped the
className. Next types CSS modules as { [key: string]: string }, so nothing
flagged it.

Also removed the dangling reference and the import that became unused with it.

Activating the rule today would be a regression. In the off-canvas menu the
two h3s are "Components" and "Integrations" — Section headings from
MainNavigation. Hiding them would drop the labels but keep the
expand-all/grouping buttons that sit on the "Components" header row, leaving two
unlabelled icon buttons. "Menü" is an h2 and unaffected. No indication UX wants
this; the desktop navigation shows the same headings.

Verification

Ran the docs app and inspected the live DOM (pnpm nx dev docs), plus the
prerendered output of pnpm nx build docs.

Check	Result
.mainContent > h3, > h4 matches	0 — headings sit in the display: contents wrapper (11 h3/h4 as grandchildren)
h3 computed margin-top	24px, before and after — from .anchorLinkHeading&h3
.mobileNavigationOffCanvas in the document	absent — off-canvas root is div.flow--overlay.flow--modal--off-canvas
Mobile menu at 900px, before vs. after	Identical
stylelint on both files	3 selector-max-type warnings → 0
pnpm lint, pnpm nx build docs	Pass

No visual test snapshots are involved — this is docs layout, not
packages/components.

For UX — not changed here

MDX ### renders as a plain Heading level={4} with no top margin (only the
16px row-gap); 5 content files use it. The deleted rule wanted 16px there, but
#2870 deliberately moved h3 to the Flow scale, where the h4 counterpart is
--section--sub-heading-spacing-s (8px), not 16px. Picking a value is UX's call,
so this PR leaves it alone.

Note for #3091

Once this lands, #3091 can drop both of its disable blocks — the warnings are
gone with the rules.

related #3091, #3021

🤖 Generated with Claude Code

[github-actions]

Coverage Report for ./packages/components/

Status	Category	Percentage	Covered / Total
🔵	Lines	78.69%	746 / 948
🔵	Statements	78.57%	763 / 971
🔵	Functions	80.09%	165 / 206
🔵	Branches	70.33%	377 / 536

File Coverage

No changed files found.

Generated in workflow #6582 for commit 5a21bce by the Vitest Coverage Report Action

[github-actions]

🚀 Preview Deployment

Preview environments are ready:

Type	URL
docs	pr-3099.docs.review.flow-components.de
storybook	pr-3099.storybook.review.flow-components.de

Images:

• docs: ghcr.io/mittwald/flow/docs:pr-3099
• storybook: ghcr.io/mittwald/flow/storybook:pr-3099