Keyboard handling issues: Shift+Tab and Alt+key on macOS

[EugenEistrach]

Two keyboard handling issues:

1. Shift+Tab doesn't send backtab sequence

Shift+Tab should send ESC[Z (backtab), but ghostty-web doesn't handle this case.

Proposed fix - add Shift+Tab handling in handleKeyDown:

if (event.shiftKey && event.key === "Tab") {
  this.onDataCallback("\x1B[Z");
  event.preventDefault();
  return;
}

2. Alt+key on macOS produces wrong characters

On macOS, Alt+letter produces transformed Unicode characters (e.g., Alt+T → †, Alt+D → ∂). The terminal should send ESC+letter sequences instead.

The issue: event.key is already transformed by macOS, but isPrintableCharacter uses it directly. The encoder then receives garbled input.

Proposed fix - intercept Alt+letter on macOS and use event.code:

if (isMac && event.altKey && !event.metaKey && !event.ctrlKey) {
  const match = event.code.match(/^Key([A-Z])$/);
  if (match) {
    const baseKey = event.shiftKey ? match[1] : match[1].toLowerCase();
    this.onDataCallback(`\x1B${baseKey}`);
    event.preventDefault();
    return;
  }
}

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Note: These fixes were developed during an AI-assisted session and are currently used in our application's keyboard handler. Due to time constraints we're unable to submit a proper PR, but wanted to share the findings.