Don't override bare Left/Right when a text input is focused (#11357)

[niksedk]

Summary

Bare Left/Right are fundamental caret navigation in every text input on every platform. The existing protection in OnKeyDownHandler only returned early when the Allow single-letter shortcuts in text box setting was off; with that setting on (or for an arrow key that the user has happened to bind), Left/Right inside the subtitle text box, the Find dialog, or any other TextBox / TextEditor / MaskedTextBox would fire the SE shortcut instead of moving the caret.

From the reporter (#11357):

When I am editing text in the text box, I want to use … LeftArrow to skip to the next/previous word. This is a global Windows standard keyboard shortcut that works in every other app.

Change

At the top of the IsTextInputFocused() branch in MainViewModel.OnKeyDownHandler (MainViewModel.cs:16764), bail out when the pressed key is bare Left or Right (no modifiers):

if ((keyEventArgs.Key == Key.Left || keyEventArgs.Key == Key.Right)
    && keyEventArgs.KeyModifiers == KeyModifiers.None)
{
    return;
}

Modified combinations (Ctrl+Left, Shift+Right, Ctrl+Shift+Left, etc.) keep going through the normal shortcut resolution — some users do bind those, and overriding them would surprise them.

The check uses the existing IsTextInputFocused() helper, so it applies broadly: subtitle text box, original text box, Find dialog, any future window with a text input.

Test plan

• In Options → Settings → Tools, turn on Allow single-letter shortcuts in text box, bind Left arrow to "Video go back 1 second", click into the subtitle text box, press Left. Confirm the caret moves left and the video does NOT move.
• Same setup, press Right. Caret moves right; video does not move.
• Same setup, press Ctrl+Left. Ctrl+Left still triggers whatever shortcut the user has bound to it (if any), matching existing behavior for modifier combinations.
• Bare Left/Right while the subtitle grid is focused still works as before (does whatever the user has bound to those keys).
• Bare Left/Right inside the Find dialog still moves the caret.
• Build clean; UI tests pass (306/306).

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