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/login.ts

@@ -1,6 +1,6 @@
-document.getElementById('f').onsubmit = async e => {
+document.getElementById('f')!.onsubmit = async e => {
   e.preventDefault();
-  const msg = document.getElementById('msg');
+  const msg = document.getElementById('msg')!;
   msg.textContent = '';
   const r = await fetch('/api/login', {
     method: 'POST',