fix(mobile): keep lesson hubbar on one line on small screens

[dineshbyte]

On narrow phones the sticky lesson #hubbar had no mobile rule, so its three flex items (back-link, Prev/Next pager, theme toggle + track name) shrank and wrapped mid-item ('Next' dropped below 'Prev', the track name wrapped) — the cramped header reported on mobile.

At <=540px: nowrap the pager, tighten gaps, and hide the redundant track-name label (the lesson title right below already names the topic). The back-link, Prev/Next, and theme toggle stay on one clean line. Shared CSS, so it fixes every lesson across all 9 tracks; hubbar markup is identical everywhere.

What & why

Type of change

• Content — new lesson / track, or reworked existing content
• Content fix — typo, clarity, accuracy, or diagram fix
• Bug fix — broken link, layout, search, etc.
• Feature — site/tooling capability
• Chore / docs / CI

Scope

Checklist

• node scripts/check-broken-links.js passes (no broken internal links)
• node scripts/validate-content.js passes (SEO tags, interview data-levels, JSON-LD, sitemap in sync)
• New/renamed pages are added to docs/sitemap.xml
• Each page has a unique <title> (· StackDepth), meta description, canonical, and Open Graph tags

For content changes

• Visual-first: the concept is carried by diagrams/tables/cards; prose only labels them (passes the "skim with text too small to read" ship test)
• Accurate & cited: non-obvious claims cite high-trust sources (RFCs, specs, vendor/standards docs) — no claims from memory
• Interview panel present where expected, with valid data-level values (core / senior / staff / design)
• Quiz options are length-balanced; dark and light modes both legible

Notes for the reviewer