This commit enables macOS dictation (triggered by pressing Fn twice) to work
in kitty by implementing the necessary accessibility methods.
The key fix is changing `selectedRange` to return `NSMakeRange(0, 0)` instead
of `kEmptyRange` (NSNotFound, 0). When selectedRange returns NSNotFound, macOS
dictation cannot determine where to insert text and fails silently.
Additional accessibility methods implemented:
- accessibilitySelectedTextRange: Returns cursor position for dictation
- accessibilityNumberOfCharacters: Returns 0 (terminal has no fixed buffer)
- accessibilityInsertionPointLineNumber: Returns 0
- accessibilityValue: Returns empty string
- setAccessibilityValue: Routes dictated text to keyboard input
This fix is inspired by the similar fix in Emacs v30 which restored dictation
by implementing selectedRange properly after migrating to NSTextInputClient.
Fixes: https://github.com/kovidgoyal/kitty/issues/3732
0bf307c95f