Avoid invalid subtitle add/remove paths near project end

[Copilot]

Repeated Add Subtitle attempts near the end of the project could leave behind an empty subtitle track after the action was rejected for insufficient space, and subtitle track removal still executed after detecting an invalid index. This change makes rejected adds a true no-op and hardens track removal against out-of-range access.

• Add Subtitle no longer creates tracks on rejection

  • SubtitlesDock::onAddRequested() now checks timing constraints before calling ensureTrackExists() when no subtitle track exists.
  • Repeated "Not enough space to add subtitle." actions no longer mutate subtitle-track state.

• Existing-track behavior stays intact

  • Duplicate detection and next-item gap calculation still run for a valid selected track.
  • Track creation is deferred until insertion is actually possible.

• Invalid subtitle track removal now exits immediately

  • SubtitlesModel::doRemoveTrack() now returns after logging an invalid index instead of falling through into beginRemoveRows() / container removal.

if ((maxTime - msTime) < 500) {
    MAIN.showStatusMessage(tr("Not enough space to add subtitle."));
    return;
}

if (trackIndex < 0) {
    ensureTrackExists();
    trackIndex = m_trackCombo->currentIndex();
}

• Fixes Crash (access violation) when Add Subtitle is repeated after "Not enough space to add subtitle" #1912

[Copilot]

Copilot review overview

🟢 Approval recommended

The reviewed changes address the identified subtitle-track state and bounds issues.

Review effort: Lite
Findings: None

What changed in this PR

Prevents invalid subtitle-track mutations when adding subtitles near the project end and safely handles invalid track removal.

Changes:

• Defers track creation until subtitle insertion is valid.
• Returns immediately for invalid subtitle-track indices.

File	Description
src/​models/​subtitlesmodel.cpp	Stops invalid track removal before model/container access.
src/​docks/​subtitlesdock.cpp	Validates available space before creating a subtitle track.

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[bmatherly]

This looks good.