test: Vitest + RTL unit tests for lib/{geo,share,places} (deferred)

[AnayDhawan]

Deferred until there's more contributor volume — tracked so it isn't lost. There are no unit tests
today, only e2e. The pure helpers in src/lib/ are easy, high-value targets.

Task

• Add Vitest + React Testing Library (RTL).
• Unit-test lib/geo.ts (distance, placesByDistance), lib/share.ts (URL encode/decode round-trip),
  lib/places.ts (filtering).

Acceptance: npm run test:unit passes with meaningful coverage of the three modules.