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/layout.html

@@ -2,6 +2,13 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <link rel="stylesheet" href="/static/app.css">
+<script>
+// preset preview hook (#16): ?theme=<name> sets data-preset for screenshots only
+(function () {
+  var t = new URLSearchParams(location.search).get('theme');
+  if (t) document.documentElement.dataset.preset = t;
+})();
+</script>
 {{end}}
 
 {{define "topbar"}}