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/gestures.ts

@@ -70,7 +70,7 @@ document.addEventListener('touchstart', ev => {
     // back button is never swallowed with it.
     if(t.clientX < 14 && narrow() && ev.cancelable) ev.preventDefault();
   }
-  else if(list && list.scrollTop <= 0 && !VIEWS[S.feed]?.url) touch = {x: t.clientX, y: t.clientY, t: Date.now(), pane: 'list', dx: 0, dy: 0};
+  else if(list && list.scrollTop <= 0 && !VIEWS[S.feed ?? '']?.url) touch = {x: t.clientX, y: t.clientY, t: Date.now(), pane: 'list', dx: 0, dy: 0};
 }, {passive: false});
 
 document.addEventListener('touchmove', ev => {