fix(frontend): empty search query causes unintended navigation and search behavior

[tkitsunai]

Bug

The search handlers in NavBar.tsx (handleSearchClick, handleSearchKeyDown,
handleExactSearchClick) embed searchString directly as a URL path segment.
Searching with an empty string breaks in two different ways depending on
whether "exact" is checked, because the route is defined as
/search/:search_string/:exact? (App.tsx):

• Without exact: navigate(`/search/${searchString}`) with an empty
  string becomes /search/. Since :search_string is a required param, this
  doesn't match the route at all and falls through to the catch-all
  (<Route path="*" element={<Legend />} />) — the user lands on the Legend
  page with no indication of what happened.
• With exact: navigate(`/search/${searchString}/exact`) with an empty
  string becomes /search//exact, which the browser normalizes to
  /search/exact. This matches the route with search_string="exact" —
  the app silently searches for the literal string "exact", and the actual
  exact-match flag is lost (:exact ends up undefined).

Root cause

Two optional/ambiguous path segments in sequence (:search_string?/:exact?
has the same problem even if both are marked optional) can't unambiguously
represent "search_string omitted, exact specified" — the resulting URL
collides with a valid search_string value. This is a structural limit of
path params here, not a one-off bug in the handlers.

Discussion: proposed fix

Move from path params to query params:

• App.tsx: route becomes /search (no params)
• Search.tsx: useParams → useSearchParams (existing react-router-dom
  API, no new dependency)
• NavBar.tsx: the three navigate() calls become
  /search?q=...&exact=true

This removes the ambiguity entirely since query params don't have positional
meaning. Open to alternative approaches if there's a preference for keeping
path params (e.g. validating/blocking empty search client-side instead) —
posting this as a discussion before implementing since it touches routing
structure, not just the buggy handlers.