Remove Dec 2024/Jan 2025 challengePeriod branches

[Saby-Bishops]

Motivation

• The Dec 2024 / Jan 2025 time-gated campaign is being retired, so the date-gated challengePeriod checks and related UI flows should be removed.

Description

• Removed the exported challengePeriod constant from src/app/shared/dialogs/dialogs-announcement.component.ts so the date gate no longer exists.
• Removed conditional branches that opened the announcement dialog when the campaign was active from courses-step-view.component.ts, exams-view.component.ts, home.component.ts, and notifications.component.ts and simplified openAnnouncementDialog to stop trying to open the campaign dialog.
• Deleted the automatic community startup campaign flow (communityChallenge) and its invocation in community.component.ts, and removed the campaign notification sender that only ran under the date-gated branch.
• Cleaned up imports/usages of the announcement dialog where it was only referenced for the expired campaign check so the code no longer depends on challengePeriod at those call sites.

Testing

• Ran npm run -s lint in the environment; it failed due to missing Angular CLI (ng: not found), so lint could not be validated here (failed).
• Ran TypeScript compile check npx tsc -p tsconfig.json --noEmit; it failed because project dependencies (Angular packages / tslib) are not installed in this environment, so a full type-check could not complete (failed).

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Codex Task