SequencePlayer: replace basic Slider with a TimelineSlider component

[Copilot]

The SequencePlayer used a plain Slider with an inline custom background. This replaces it with a dedicated TimelineSlider component that looks and behaves like a real timeline.

Description

Introduces TimelineSlider, a Slider subclass with timeline-style visuals, and wires it into SequencePlayer.

Features list

• Playhead handle — downward triangle + vertical line (palette.highlight) replaces the default circular thumb
• Ruler — frame-number labels and tick marks above the track; major-tick interval adapts to range (1 / 5 / 10 / 50 / 100 / 250)
• Taller track — 8 px (up from 4 px), rounded ends
• Cached-frame highlighting — blue intervals preserved, extracted into a cachedFrames property on the component
• TimelineSlider registered in Controls/qmldir for reuse

Implementation remarks

TimelineSlider keeps the same public interface as Slider (from, to, value, stepSize, snapMode, live, pressed) so the drop-in replacement in SequencePlayer requires no logic changes — only the inline background/cacheView block is removed.

The cached-frame x-position formula (modelData.x * frameLength) is intentionally unchanged from the original: cachedFrames carries 0-based frame indices relative to sequence start, matching frameRange.min = 0 in the existing model.

The ruler tick interval is chosen so there are ~10–15 major ticks regardless of sequence length, preventing label crowding without requiring dynamic width measurement.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.36%. Comparing base (9bed2ab) to head (d2444c8).
⚠️ Report is 2 commits behind head on develop.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3059      +/-   ##
===========================================
- Coverage    83.38%   83.36%   -0.03%     
===========================================
  Files           73       73              
  Lines         9874     9880       +6     
===========================================
+ Hits          8233     8236       +3     
- Misses        1641     1644       +3     

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