fix: prevent keyboard shortcuts from capturing input in diff commit textbox

[matt2e]

Summary

• Add isContentEditable check to keyboard event handlers in DiffModal.svelte and diffKeyboard.ts so that typing in the contentEditable commit textbox is not intercepted by diff navigation shortcuts (e.g., 'a' for AI annotations, arrow keys for navigation)

Test plan

• Open the diff modal and verify the commit textbox is typeable (all characters including 'a')
• Verify AI annotation reveal (hold 'a') still works when focus is outside the textbox
• Verify diff keyboard navigation still works when not focused on an input

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