[codex] Harden GoRouter route argument parsing

[jjoonleo]

Summary

• add defensive typed route argument parsing for /calendar, /scheduleStart, and /earlyLate
• preserve valid existing extras while adding durable query parameters for calendar and early/late routes
• redirect malformed /earlyLate payloads to /home instead of crashing
• cover missing, malformed, legacy, and query-based route payloads with focused tests

Why

Direct URL entry, browser refresh, stale links, or native/deep-link payload drift can drop or corrupt state.extra. The previous direct casts could throw in production before the user reached a recoverable state.

Closes #391

Validation

• flutter test test/presentation/shared/router/route_arguments_test.dart
• flutter test test/presentation/alarm/screens/preparation_flow_widget_test.dart
• flutter analyze
• git diff --check

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