The attachCustomKeyEventHandler in ghostty-web returns:
- true: PREVENT default handling (we handled it)
- false: ALLOW default handling (let ghostty process it)

The clipboard shortcut handler (#1563) had this inverted, returning true
for all non-keydown events and at the end of the handler, which prevented
ghostty from processing any keyboard input.

Fixes terminal input regression introduced in d3928b5.

…1586)

## Summary

Fixes terminal input regression introduced in #1563 (d3928b5).

## Root Cause

The `attachCustomKeyEventHandler` in ghostty-web returns:
- `true`: **PREVENT** default handling (we handled it)
- `false`: **ALLOW** default handling (let ghostty process it)

The clipboard shortcut handler had this inverted:
- Returned `true` for all non-keydown events → blocked keyup/keypress
- Returned `true` at end of handler → blocked all normal key input

This effectively disabled all terminal input except the specific
copy/paste shortcuts.

## Fix

Invert all return values to match ghostty-web's API contract.

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