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>

tsconfig.json

@@ -5,14 +5,9 @@
     "target": "es2022",
     "lib": ["es2022", "dom", "dom.iterable"],
     "noEmit": true,
-    // strict, a flag at a time (#235); strictNullChecks still to come, with a typed $ (#236).
-    "strict": false,
-    "noImplicitAny": true,
-    "strictFunctionTypes": true,
-    "strictBindCallApply": true,
-    "noImplicitThis": true,
-    "alwaysStrict": true,
-    "useUnknownInCatchVariables": true,
+    // Strict throughout (#235, #236). $ assumes the element is there: the page check proves the
+    // ones wired at load exist, and a lookup that may find nothing says so with ?.
+    "strict": true,
     "skipLibCheck": true
   },
   "include": ["web/src/*.ts"]