v0.4.0 Wave 20: custom layouts + rehearse timings (final v0.4.0 wave)

[900Labs]

Summary

Wave 20 completes the v0.4.0 milestone with two features: a layout picker (building on the Wave 9 template system) and rehearse timings with auto-advance.

What changed

Model (slides-core)

• Slide.rehearsed_duration_ms: Option<u32> (additive). SetSlideRehearsedDuration command (reversible).

Desktop (apps/desktop/)

• Layout picker: dropdown in the slide context menu showing the current template's layouts. Selecting one calls set_slide_layout and re-renders placeholder guides.
• Rehearse mode: "Rehearse" button in the presenter records per-slide durations. "Done" commits them via set_slide_rehearsed_duration.
• Auto-advance: "Use timings" toggle advances automatically after each slide's rehearsed duration, with a countdown indicator.
• set_slide_rehearsed_duration Tauri command (transactional). Zero @html.

Review process

Control-tower pipeline applied. Review agent: all 7 checks CLEAN. Two minor NITs (N undo entries per rehearsal; theoretical auto-advance re-entry) — both acceptable.

Test results

• 374 workspace tests pass (slides-core +1).
• npm run check (svelte-check): 0 errors, 0 warnings.
• scripts/verify-local.sh: green. scripts/verify-public-release.sh: green.

v0.4.0 complete

This PR closes the v0.4.0 milestone. All roadmap items shipped across waves 16-20.