fix: safe router navigation encoding and /users/search test coverage

[Copilot]

Two router.push calls in the messages tab interpolated usernames directly into path strings, breaking navigation for usernames containing reserved URL characters. The /users/search endpoint also lacked any test coverage.

Changes

• frontend/app/(tabs)/message.tsx — Replace string-interpolated router.push with the object form in both handleSelectSuggestion and handleStartChat:

  // Before
  router.push(`/conversation/${username}`);
  
  // After
  router.push({ pathname: "/conversation/[username]", params: { username } });

  Consistent with the existing pattern in _layout.tsx and MessageThreadItem.tsx.

• backend/api/internal/tests/main_test.go — Register GET /users/search on the test router so the endpoint is reachable in tests.

• backend/api/internal/tests/user_test.go — Add three test cases for /users/search:

  • Empty q → []
  • Prefix match (q=dev) → returns dev_user1
  • count exceeding 20 is capped (q=d&count=100) → returns both matching users

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[grillinr]

Closes #129