chore(frontend): remove orphaned duplicate singletons ObserverPatterns + CacheManager (#12103, #12104)

[mrveiss]

Closes #12103
Closes #12104
Part of #11682

Thinking Path

Owner rule for orphans: if it's a duplicate already replaced with no functionality lost → remove; otherwise wire in. Both qualify for removal (verified 0 imports, 0 named-export usage, no .d.ts, no barrel re-export):

• ObserverPatterns.js (eventObserver/stateObserver + 6 exports): pub-sub is already provided by the canonical useEventBus (subscribe/unsubscribe) + LiveEventService; state-watch by the 11 Pinia stores. Duplicate of already-replaced functionality, 0 consumers → no loss.
• CacheManager.ts (cacheManager browser-cache clearing): superseded by CacheBuster.js (wired into App.vue's clearAllCaches). Duplicate, 0 call-sites → no loss.

(The 3rd orphan, AdvancedControlApiClient #12102, is NOT removed — its backend routes are live and it's the client for the #11506 desktop-takeover feature; it needs wiring in, tracked separately.)

Verification

Zero imports of either file / their named exports across src (grep incl .d.ts, barrels, dynamic). No references remain post-removal. Build-safe (nothing depended on them).

Model Used

Claude Opus 4.8
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[github-actions]

✅ SSOT Configuration Compliance: Passing

🎉 No hardcoded values detected that have SSOT config equivalents!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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