fix: reduce change detections

[arturovt]

Reduced Angular change detection triggers from ~5+ per HTTP request down to 1 (or 0 if no observers).

Key optimizations:

• Wrapped HttpClient calls in runOutsideAngular() to prevent zone.js from triggering CD on XHR progress events (send, download, load, etc.)
• Re-enter Angular zone with ngZone.run() only when emitting to @output() with active observers
• Added AbortController to make async render operations cancellable (Promises are not cancellable by default in JavaScript)
• Used switchMap to automatically cancel stale in-flight requests when src changes rapidly

Performance impact:

• Before: HTTP request triggers 5+ CD cycles (one per XHR event)
• After: HTTP request triggers 1 CD cycle (only on final emission, and only if observers exist)

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Note: Also updated demo app during debugging session. The demo had scroll event handlers triggering redundant change detections, which made it harder to profile the markdown component's actual CD behavior. These demo changes aren't strictly necessary for the optimization but were helpful for accurate performance measurement.

[arturovt]

@jfcere please note that some formatting changes have been done automatically, I was trying to revert them, but they're still being formatted once I click save.