Add 42 curated themes from Figma's website color schemes

Sourced from Figma's "53 Unique Website Color Schemes" resource page,
which only shows rendered mockup screenshots (no raw hex data) - extracted
dominant colors via canvas pixel sampling, then a small pipeline
(tools/generate_website_scheme_themes.py) maps each scheme's real colors
into this app's palette shape, with contrast enforced against both the
background and the white text every .btn uses.

Cut 11 of the original 53 after review: 2 for a neon-bright accent (the
saturation cap only bounded lightness at first, so ~28 of 53 accents
landed at pure 100% saturation - fixed at the pipeline level, calibrated
against what the existing hand-picked themes actually use), and 9 for
being near-duplicates of another kept scheme once mapped into actual UI
colors (measured by real color distance, not eyeballed) - including one
pixel-identical pair and a 4-way cluster trimmed to its two most distinct
members.

Settings dropdown is grouped into <optgroup>s (Base, Editor Themes, and
5 categories matching the source page's own curation) rather than one
flat 54-option list.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/settings.html

@@ -348,8 +348,12 @@
                     <span class="setting-desc">Overall light/dark palette</span>
                 </label>
                 <select id="theme" data-setting="theme">
-                    {% for choice in theme_choices %}
-                    <option value="{{ choice }}" {% if settings.theme == choice %}selected{% endif %}>{{ theme_display_names[choice] }}</option>
+                    {% for group_label, choices in theme_groups %}
+                    <optgroup label="{{ group_label }}">
+                        {% for choice in choices %}
+                        <option value="{{ choice }}" {% if settings.theme == choice %}selected{% endif %}>{{ theme_display_names[choice] }}</option>
+                        {% endfor %}
+                    </optgroup>
                     {% endfor %}
                 </select>
             </div>