Page type-check: strict throughout, a typed $, null checks (#236)

$<T> returns the element type it is given and assumes the element is there: 122 lookups take
their type from the tag their id is on in the markup. A handler reads its target through a
cast, since a typed element will not take a handler whose event names a narrower target.
strictNullChecks found no bug the page had; its 94 places were values the code already handles
in ways the type-check could not follow (data- attributes the page writes, a map entry set the
line before, Share drawn only for an item with a link), each now said so.

isPlayable stays a plain boolean: as a type guard its false would have meant "no file", and it
is also false for a file that is not audio or video.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/events.ts

@@ -26,7 +26,7 @@ function connect(){
       // Only the row actually downloading. Without the enclosure id this used to paint
       // every pending bar at once, so adding a feed looked like it was fetching the lot.
       const bar=document.querySelector<HTMLElement>(`.dlbar[data-bar="${ev.enclosure}"] i`);
-      if(bar){ bar.style.width=pct+'%'; bar.parentElement.classList.add('live'); }
+      if(bar){ bar.style.width=pct+'%'; bar.parentElement!.classList.add('live'); }
       $('#count') && ($('#count').textContent=`downloading ${ev.file} — ${pct.toFixed(0)}%`);
     }
     else if(ev.ev==='download_done'){