Fix change frame rate/speed corrupting selected line duration (#11650)

[niksedk]

Fixes #11650.

Problem

When changing frame rate (e.g. 25 → 23.976), the currently selected line gets a corrupted end time / duration — sometimes by a few seconds, sometimes by minutes/hours. Every other line scales correctly.

In the reporter's screenshots, line 9 (selected) scales its start correctly (01:05:48,320 → 01:08:36,950) but its end jumps to 01:11:32,782, giving a ~2m55s duration instead of ~1.9s.

Root cause

ChangeFrameRate (and the identical ChangeSpeed) updated each line's times in two separate steps — start first, then end:

line.SetStartTimeOnly(line.StartTime.TotalMilliseconds * ratio);
line.EndTime = line.EndTime.TotalMilliseconds * ratio;

When times grow (ratio > 1), step 1 moves the start past the still-unscaled end, momentarily producing start > end → a negative duration, which is published via change notification.

That transient is harmless for ordinary rows, but the selected row is live two-way-bound to the editor's time/duration up/down controls. The duration control (SecondsUpDown) coerces the negative value to zero and writes it back into the model, which fires OnDurationChanged and moves the end time. Interleaved with step 2, the selected line's end ends up scaled roughly twice (4292.78 ≈ 3950.2 × ratio²). Non-selected rows have no bound controls, so they are unaffected.

Fix

Add SubtitleLineViewModel.SetTimes(start, end) which assigns both times with change notifications suppressed (the same idiom already used by the constructor and UpdateFrom) and recomputes duration once — so no invalid intermediate state is ever exposed to the bound controls. Use it from both ChangeFrameRate and ChangeSpeed (which had the identical latent bug).

Testing

• dotnet build src/ui/UI.csproj succeeds with 0 warnings / 0 errors.

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