Filter cheatsheet dropdown and shortcuts from search field

[federunix]

Summary

• Extend the search field to filter the cheatsheet dropdown by application name (substring match, including space/hyphen variants) while keeping manual selection via the dropdown.
• Support multi-word queries: leading terms select a cheatsheet (e.g. vim), remaining terms filter shortcuts within that sheet (e.g. vim copy).
• Update keyboard shortcuts help in the built-in keyhint cheatsheet and clarify search/dropdown tooltips.

Test plan

• Launch keyhint and confirm the cheatsheet dropdown lists all applications on startup
• Type vim in the search field and verify the dropdown narrows to vim and vim-related cheatsheets
• Type vim copy and verify the vim cheatsheet is shown with only copy-related shortcuts
• Type copy alone and verify shortcuts are filtered on the current sheet while the dropdown still lists all cheatsheets
• Use Ctrl+Up / Ctrl+Down to cycle through filtered cheatsheets
• Clear search with Esc, Ctrl+Backspace, or Ctrl+u and confirm the initial auto-detected cheatsheet is restored
• Pick a cheatsheet manually from the dropdown after filtering