Improve and fix searching behaviors

[denkeni]

1. Tapping board search result will shop board page, based users' feedback.
2. Fix search results may be incorrect when changing keywords (race condition), as attached.

https://github.com/Ptt-official-app/Ptt-iOS/assets/2727287/3dd8e94e-5803-4975-8178-3c4964c5697c