[steventeng2022]

Adds a Find nearest station / 定位最近的車站 button to the metro dashboard. For example, a rider in Kaohsiung with Taipei Metro selected can locate once to switch to the nearest station's system and serving line, open its map, and load its arrival information.

• Compare stations across all metro systems using the existing cached repository and shared location resolver. Calculate distances locally and request location only after a tap.
• Resolve line membership from station-of-line, including stations whose line_id is empty. Ignore invalid coordinates and unavailable lines; prefer the current line at an interchange when distances tie.
• Keep usable results when a system fails and disclose the incomplete coverage. Preserve the current selection on failure, and discard late results after a manual selection or tab change.
• Include English and Traditional Chinese strings, generated localizations, README documentation, and unit/widget regression coverage.

Closes #55.

Validation with Flutter 3.47.6 / Dart 3.13.5:

• 26 focused tests passed across nearest_metro_station_test.dart, metro_nearest_station_test.dart, user_location_test.dart, metro_direction_test.dart, and metro_direction_selector_test.dart.
• Static analysis of all changed Dart files: no issues.
• Formatting and git diff --check: passed.

The repository-wide analyzer reports six pre-existing unawaited_return_in_try_block warnings in app_controller.dart, app_update_service.dart, and bus_repository.dart with this newer local SDK. The repository's CI uses Flutter 3.41.6; no dependency or SDK versions are changed by this PR. Physical-device GPS and the full test suite were not exercised locally.

[Av1anJay]

Code review: changes requested

• lib/screens/metro_dashboard_screen.dart:262-263 — Incomplete system data is reported as complete. A system is counted as available whenever its three requests return successfully, even if its stations or station-of-line list is empty, or all its stations have invalid coordinates / no matching line. For example, if KRTC returns HTTP 200 with an empty station list while TRTC has data, a user in Kaohsiung will be told that a Taipei station is the nearest with no incomplete-coverage warning. _getJsonList also converts a non-list HTTP 200 response to an empty list, so this can occur without an exception. Detect systems without usable candidate stations as incomplete, and add a widget regression test with one unusable and one usable system.

Checks: GitHub CI Verify passed (Flutter analyze, full Flutter tests, web route test, Node runtime tests); Android, iOS, Linux, macOS, Windows, Wear OS and Web builds passed. Locally, git diff --check passed and the Node runtime tests passed on Node 22.23.4 (3 tests). Flutter SDK is not installed locally, so Flutter checks were not rerun here. Inspected live metro API responses and reproduced the incomplete-coverage condition with an empty KRTC station list.