Extract PromptSaveChanges helper for save-changes dialogs

[niksedk]

Follow-up to #11985.

The five "Save changes?" Yes/No/Cancel prompts in MainViewModel each repeated the same show dialog → decide → maybe save block. This extracts a single PromptSaveChanges helper (plus SaveCurrentSubtitle / SaveCurrentSubtitleOriginal) so the dialog and — importantly — the cancel/dismiss handling live in one place:

private async Task<bool> PromptSaveChanges(string promptText, Func<Task<bool>> save)
{
    var dr = await MessageBox.Show(Window!, Se.Language.General.SaveChangesTitle, promptText,
        MessageBoxButtons.YesNoCancel, MessageBoxIcon.Question);

    if (dr == MessageBoxResult.Yes)
    {
        return await save();        // proceed only if the save succeeded
    }

    return dr == MessageBoxResult.No; // No → proceed; Cancel / None (dismissed) → abort
}

Behavior is unchanged:

• Yes → save, proceed only if the save succeeded (so a cancelled "Save as" still aborts).
• No → discard and proceed.
• Cancel, or dismissing via the title bar (MessageBoxResult.None) → abort.

The two close handlers (OnClosing, OnWindowClosing) keep calling SaveSubtitle() directly as before (via the inline save lambda); the three text-flow sites keep the save-as-if-untitled logic via the small SaveCurrent* helpers.

Net −36 lines, and the None (dismissal) handling is no longer duplicated across five call sites.

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