feat(overview): sticky eyebrow + bigger green nav + keyboard & swipe nav

[HenryLach]

Three layered improvements to the slide-deck navigation UX, all landing in one PR since they reinforce each other.

1. Sticky eyebrow header

.eyebrow now uses position: sticky; top: 0 so the title, wordmark, and prev/index/next controls dock to the top of the viewport on both desktop and mobile once the page is scrolled. Sticky (not fixed) keeps it in normal flow until it hits viewport top, so there's no body padding-top compensation and no content jumping. Background fills with --bg, plus a 1px border-bottom for separation when scrolled.

2. Larger, green nav controls

• .page-nav font-size doubles (calc(var(--fs-lg) * 2)) so the prev/index/next glyphs read as primary actions instead of subtle hints.
• Color switches from --text-faint to --green (matching the eyebrow title color).
• Hover bumps to --text with a tiny translateY(-1px) for tactile feedback.
• Each control has a 44×44 minimum hit area via min-width / min-height / padding, satisfying touch-target guidance on phones.
• Disabled state stays gray + low-opacity so first-page-prev and last-page-next still read as inactive.

3. Keyboard + swipe navigation (nav.js)

New shared script, ~3 KB, vanilla browser APIs only, no dependencies. Loaded by every page via <script src="nav.js"></script> before </body>.

Keyboard: ArrowLeft / ArrowRight synthesize a click on the existing prev / next anchors. Ignores modifier-key shortcuts and editable targets (input, textarea, contenteditable) so it never hijacks normal browser shortcuts or form input.

Touch: horizontal swipes (≥70px X movement, and at least 2× more X than Y) fire prev / next. Unless the swipe started inside a horizontally scrollable container (e.g. the page-09 git-graph card or the page-08 wave cards) — in which case the inner pan keeps working. The check walks up the DOM ancestor chain looking for any element with scrollWidth > clientWidth.

No-op on the index page (no .page-nav element to find), safe to load everywhere.

To verify before merging

Same three checks on the PR branch:

• Desktop: scroll any page → eyebrow docks at top. Click anywhere outside a form, hit ← and → to navigate. Buttons should be visibly green and roughly 2x the previous size.
• Phone: swipe left/right anywhere except on page 09's git-graph card and page 08's wave cards — those should still pan horizontally inside without triggering page nav.
• Page 09 + 08: confirm the special horizontal-scroll areas still work (swipe inside them moves the inner content, not the page).

No release. Pure docs/microsite enhancement.