v2 Phase 1 — Mobile-first polish (sticky preview, sheets, spacer widget, PWA)

[szrudi]

Umbrella issue for Phase 1 of the v2 UI overhaul: a coherent mobile-first pass — preview stays visible while editing, settings/batch become sheets on mobile, the batch table becomes touch-friendly, and we add the first new widget (Spacer) to validate the widget-add pipeline before Phase 2 leans on it.

Builds on Phase 0 (#27).

Slices

1.1 — Sticky preview + sticky print bar (M)

• Restructure App.tsx so the preview stays visible while editing on mobile.
• PrintButton becomes a sticky bottom action bar on mobile.
• Desktop two-column layout preserved via lg: ordering.
• Done when: on 390×844 the live preview is visible while typing in any widget editor.

1.2 — Settings + Batch as bottom sheets (M)

• Replace inline <details> with a bottom-sheet/drawer pattern below md:.
• Above md: keep current inline behavior.
• Build one reusable <Sheet> primitive (native <dialog> + ~50 LOC; no library).
• Settings sheet groups: Tape & layout / Colors / Printer (clearer than the 7-wide flex-wrap row).
• Done when: on mobile both Settings and Batch open as full-width sheets with a clear close affordance; desktop unchanged.

1.3 — Mobile-friendly Batch table (M)

• Below md:, render each batch row as a stacked card (one input per variable per row) instead of a horizontal-scrolling table.
• Promote per-row delete + preview-eye to tappable icons.
• Above md: keep the table.
• Done when: 390 px viewport with 3 variables × 4 rows has no horizontal scroll and no input overlap.

1.4 — New widget: Spacer / divider (M)

• New SpacerWidget type with widthPx and style (blank / vertical-line / dotted).
• Tiny editor (slider + style toggle).
• Backend: render with a small new render engine — start with a 30-min spike to confirm labelle accepts a raw bitmap engine or we drop in a thin one.
• Validates the widget-add pipeline end-to-end as a stepping stone for Phase 2 widgets.
• Done when: [Text][Spacer 20px][QR] shows a clear gap and round-trips through save/load.

1.5 — PWA polish (S)

• Maskable icon variants, mobile chrome theme match, dismissible "add to home screen" hint on first mobile visit.
• Honest offline state in LabelPreview when the server is unreachable (NetworkFirst SW falls back gracefully).
• Done when: app installs to iOS/Android home screen, opens standalone, shows a recognisable offline state if the Pi is down.

Phase exit criteria

All slices merged, mobile smoke at 390×844 / 768×1024 / 1280×800 clean, hardware print test for the Spacer widget on hector passes.