ui: make ultra mode search live and regex-aware

Replace the Enter-to-apply search with live filtering: every keystroke
recompiles the regex and rebuilds ultraFiltered immediately, so results
update as the user types.

Behaviour summary:
- Typing: regex compiled and filter applied on each character.
- Invalid regex mid-type: previous filter kept unchanged (no flicker).
- Enter: confirm and close the input (empty Enter clears the filter).
- Esc: cancel, clear the filter, close the input.
- Regexes are always supported (was already the case via
  compileAndCacheRegex, now usable interactively as you type).

Extracted ultraApplySearch() to centralise the compile+filter logic.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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