fix: Share button was never disabled for large documents

Two places unconditionally set shareBtn.disabled = false (loadExample
and init block), overriding checkShareability(). Now the button starts
disabled and is only enabled by checkShareability() after a successful
render proves the URL fits within 2000 chars.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dvhthomas

static/index.html

@@ -918,7 +918,6 @@
     var res = await fetch(EXAMPLES_BASE + name + '.cm');
     if (!res.ok) throw new Error();
     input.value = await res.text();
-    shareBtn.disabled = false;
     lastRenderedSource = '';
     lastRawSource = '';
     renderCurrent();
@@ -950,7 +949,6 @@
     await loadFromHash();
   }
   if (input.value.trim()) {
-    shareBtn.disabled = false;
     if (location.hash && location.hash.startsWith('#0:')) {
       cmBtn.style.display = '';
     }