perf(frontend): fix slow UI rendering after /api/view response

[wcypierre]

Summary

• OnPush change detection on AppComponent and TorrentComponent — Angular now only re-evaluates components when @Input references change or markForCheck() is called, instead of on every mouse move, timer tick, and click event across the full component tree
• Eliminate duplicate torrentSearch pipe call — the footer torrent count was re-running the O(n) filter independently of the main list render; a filteredCount property is now computed once in TypeScript whenever torrents update or the search term changes
• Fix orderBy pipe in-place mutation — values.sort() was silently mutating this.torrents when no search term was active; changed to [...values].sort()
• mergeMap → switchMap for polling — cancels in-flight /api/view requests when a new poll fires before the previous response returns

Root cause

The /api/view API call was fast. The bottleneck was Angular's default change detection: without OnPush, the entire component tree (including all N torrent cards and their 7 pipes each) was re-evaluated on every browser event and every 2-second timer tick. The duplicate pipe call compounded this by running the array filter twice per cycle.

Test plan

• Run ng serve and open the app with 10+ torrents active
• Open DevTools → Performance tab, record a few seconds — verify CD runs fire only on 2-second poll ticks, not on mouse moves
• Verify the footer torrent count matches the visible list count, including when typing a search term
• Change sort field and verify list re-orders correctly (no mutation side-effects)
• Pause/resume a torrent and verify its state updates correctly (tests the markForCheck() in refreshAfter)

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