Palettes: pastel-lavender/peach/cloud presets + semantic status colors (#16)

- Three new light [data-preset] blocks alongside midnight (default) and smooth
- --ok/--warn/--err semantic tokens with 4.5:1 contrast per preset
- .toast success/error variants wired in new.html
- ?theme= query param preview hook in layout head (screenshots only)
- Preview screenshots in docs/palette-previews/

Closes #16

web/templates/new.html

@@ -74,10 +74,13 @@ function updateGutter() {
 content.addEventListener('input', updateGutter);
 updateGutter();
 
-function toast(msg) {
+function toast(msg, kind) {
   let t = document.querySelector('.toast');
   if (!t) { t = document.createElement('div'); t.className = 'toast'; document.body.appendChild(t); }
   t.textContent = msg;
+  t.classList.remove('success', 'error');
+  if (kind === 'success') t.classList.add('success');
+  if (kind === 'error') t.classList.add('error');
   t.classList.add('show');
   clearTimeout(t._h);
   t._h = setTimeout(() => t.classList.remove('show'), 2000);
@@ -162,12 +165,13 @@ async function create() {
   const data = await res.json();
   if (!res.ok) {
     showResult('Error: ' + (data.error || res.status), true);
+    toast('Create failed', 'error');
     return;
   }
   const url = location.origin + '/' + (data.custom_slug || data.id);
   showResult('<a href="' + url + '">' + url + '</a>', false);
   $('result').dataset.token = data.deletion_token || '';
-  try { navigator.clipboard.writeText(url); toast('Copied'); } catch(e) {}
+  try { navigator.clipboard.writeText(url); toast('Copied', 'success'); } catch(e) {}
   const dest = '/' + data.id + '?created=1&token=' + encodeURIComponent(data.deletion_token || '');
   // password-protected: unlock now with the password we already have (#26)
   if ($('haspw').checked && data.id) {