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>

README.md

@@ -120,7 +120,12 @@ silently stay blank instead of erroring.
 - Per-course study guide export (markdown, organized by section)
 
 **Personalization** (Settings)
-- Built-in themes or a custom accent color, corner radius, and card style
+- 54 built-in themes (grouped in the dropdown: Base, Editor Themes, and 5
+  categories of website-inspired schemes sourced from Figma's "53 Unique
+  Website Color Schemes" - 42 of the 53 kept, after cutting a couple of
+  neon-bright ones and several that ended up near-duplicates of another
+  scheme once mapped into actual UI colors), or a fully custom accent
+  color, corner radius, and card style
 - Font, text size, page width, and spacing density
 - Default library path override + manual "Refresh Library" (bypasses the
   5-minute filesystem-scan cache)
@@ -188,3 +193,16 @@ folder and file names.
 | Documents | `.txt` `.md` `.html` `.htm` `.pdf` `.docx` `.doc` `.rtf`            |
 | Subtitles | `.srt` `.vtt` `.ass` `.sub` `.sbv`                                  |
 | Quizzes   | Any doc file whose name contains `quiz`, `exam`, `test`, `assessment`, `exercise`, `assignment`, or `homework` |
+
+---
+
+## Tools
+
+`tools/generate_website_scheme_themes.py` is the one-off script that
+produced the 42 "website scheme" theme entries in `THEME_PALETTES` (of
+53 candidates - 11 were cut after visual review, 2 for a neon-bright
+accent and 9 for landing too close to another kept scheme once mapped
+into actual UI colors) - not part of the running app, kept for reference
+if those palettes ever need regenerating or extending. See its module
+docstring for how it turned Figma's color-scheme screenshots into actual
+UI palettes.