test(competitions): add end-to-end multi-league scenario coverage

[aalonsolopez]

Problem or opportunity

Multi-league competition behavior crosses domain, persistence, runtime, data, and frontend boundaries, so isolated unit tests are not enough.

Proposed solution

Add Rust integration tests plus Vitest coverage for save reload, background simulation, region selection, generated calendars, manual overrides, frontend browsing, and academy movement.

UX impact

Affected area: Rust integration test suites and frontend Vitest suites.

Acceptance criteria

• Reload scenario covers active and background competitions.
• Background simulation scenario covers due fixtures in another competition.
• Region selection scenario covers LEC and CBLOL.
• Generated calendar scenario covers deterministic reload-stable fixtures.
• Academy move scenario covers promote/demote consistency.
• Frontend scenario covers competition selection/browser rendering.

Scope

CI / release

Dependencies

Depends on all prior multi-league competition tasks through frontend/manual event foundation.

Out of scope

New production behavior not already covered by earlier issues.

Test notes

This issue is verification-focused; add missing end-to-end tests only after underlying implementation issues are available.

Approval pre-flight

• I searched existing issues and did not find a duplicate.
• I understand maintainers must add status:approved before implementation starts.
• I will branch from development using type/lowercase-slug if this is approved.
• I identified whether this changes docs, release behavior, licensing, or data provenance.

[NicoRuedaA]

WIP