[bri-b-dev]

Summary

Part A of UX-06 (live mode, completion sheet, undo and confetti follow in part B).

• Sticky head: title, category / duration / difficulty chips, favorite and the primary actions Starten and Ergebnis eintragen. While a session runs it shows the state plus Abbrechen / Speichern, so saving is always within reach. The header height is published as --app-header-h so the bar sits right below the app header
• Sections as tabs: Aufbau (diagram first, then equipment, place, setup text), Ablauf, Ziele (targets table + short stats), Verlauf. Proper tab semantics, roving tabindex, arrow keys / Home / End
• Collapsed extras: "Zu leicht? / Varianten" (the variants were not shown at all before) and the tip
• Quick entry ("Ergebnis eintragen"): open + save = 2 taps, no timer, the value field is focused; the saved session carries no timerUsed
• Starten switches to the Ablauf tab and starts the timer
• Guided sessions embed the detail without the sticky head (it would collide with the session bar)

Not in this PR (part B)

Full-screen live mode with big timer and +/- counters, keep-awake, completion sheet (level, distance to the next level, best / MA(5), PB confetti, note, Nochmal / Nächster Drill / Fertig), undo toast.

Test plan

• npm test (250), npm run lint, npm run typecheck, npm run build (local)
• CI
• Manual: header stays visible while scrolling on a phone (below the app header, no overlap); tabs by touch and keyboard; Starten -> Ablauf + timer; Ergebnis eintragen -> number keyboard, save; a drill inside a guided session; light/dark

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