feat(keybindings): add search to keyboard shortcuts dialog

[neumie]

Summary

• Add search/filter to the keybindings help dialog — type to instantly filter by action name, description, category, or keystroke

Changes

• Add search_query field and SEARCH_CHARS constant to KeybindingsHelp
• Render search_input_area between the header and scrollable content (reuses existing shared component)
• Add on_key_down handler that pushes/pops characters into the search query when not recording a keybinding
• Filter categories and actions in render() using case-insensitive substring matching
• Escape clears the search first, then closes the dialog on second press

Test plan

• Open keyboard shortcuts dialog (Cmd+K Cmd+S) — search bar visible at top
• Type "split" — only split-related keybindings shown
• Type "cmd" — keybindings with cmd in their keystroke shown
• Press Escape — search clears, all keybindings visible again
• Press Escape again — dialog closes
• Click a keybinding to record — typing goes to recording, not search
• After recording completes, typing resumes filtering