fix: move search syntax help to header, drop confusing prefix example
move [?] button from search box to header row as a subtle circled ?
next to [src] and stats. removes the "pyth → prefix on last word"
example that was unclear — tooltip now shows 3 rows: OR between terms,
phrase match, explicit OR.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>