feat: optionally show next week after the current week ends

[YYiChen]

Summary

• Add an opt-in experimental setting to show next week's timetable when the current week has no remaining classes.
• Share one WeekDisplayResolver between the main schedule and all Android widget data loaders.
• Keep manual Today widget date navigation and current-week navigation semantics intact.

Behavior

• The setting is off by default and is available under General settings > Lab.
• Saturday and Sunday only switch after checking every remaining course in the current week, including weekend courses.
• Course end times use the timetable's effective time; unparseable times conservatively prevent early switching.
• Status labels include Next week, Weekend, and This week has ended.
• The current main branch has no reschedule/make-up course date mapping model, so this PR intentionally uses stored course dates and does not add a reschedule system.

Verification

• Focused WeekDisplayResolver and widget contract tests passed.
• assembleDebug passed.
• Full test task was run; 28 existing WebView/source-contract tests fail because their relative source lookup resolves from the module working directory, unrelated to this change.
• lintDebug was run; it reports existing project-wide issues (38 errors, 427 warnings), with no findings in the changed feature files.