feat(themes): Omarchy catalog, 12 light themes, Dark/Light Appearance picker

Expand the catalog from 11 themes (10 dark, 1 light) to 37 (24/13):
all 18 remaining Omarchy defaults (palettes from each theme's
colors.toml) plus 8 canonical light themes (Tokyo Night Day, Solarized
Light, One Light, Ayu Light, GitHub Light, Gruvbox Light, Everforest
Light, Kanagawa Lotus). Lupine leads the light list as the default
light theme; the app default stays tokyo-night.

Appearance panel: Dark/Light segmented filter (SSR-initialized from the
user's current theme), scrollable theme list, Bookshelf section pinned;
active row revealed on open. Light themes carry color-scheme: light and
the soft-shadow token set. Public-page theme selects get Dark/Light
optgroups. New moon/sun icons.

tailwind.config: safelist the new theme ids — Tailwind v3 purges custom
@layer base class selectors that appear in no scanned content file, so
an unsafelisted id silently ships without its CSS (found the hard way).

theme.ts: applyTheme now swaps only theme-* classes (changing theme
from the login select no longer wipes min-h-screen) and keeps the class
in sync on <html> for the new ThemeFlashGuard — a pre-paint inline
script on the four public pages that applies the saved theme before
first paint.

Kanagawa's UI accent uses crystalBlue (#7e9cd8) instead of Omarchy's
foreground-as-accent so links stay distinguishable from text.

docs/user/themes.md

@@ -2,7 +2,10 @@
 
 ## Color Themes
 
-Bookhoard includes multiple color themes to suit your preferences:
+Bookhoard includes 37 color themes — 24 dark and 13 light — including the
+full set of [Omarchy](https://omarchy.org) defaults alongside the classics:
+
+**Dark**
 
 - **Tokyo Night** (default) - Dark blue/purple tones
 - **Dracula** - Dark purple accent colors
@@ -14,13 +17,45 @@ Bookhoard includes multiple color themes to suit your preferences:
 - **Catppuccin Mocha** - Soothing pastel dark theme
 - **Catppuccin Macchiato** - Warm mid-tone theme
 - **Catppuccin Frappé** - Frothy cool tones
-- **Catppuccin Latte** - Warm light theme
+- **Vantablack** - Pure black
+- **Last Horizon** - Near-black with warm rose accents
+- **Ethereal** - Deep midnight blue with peach text
+- **Hackerman** - Terminal green on black
+- **Matte Black** - Flat black with amber accents
+- **Solitude** - Quiet blue-grey monochrome
+- **Retro 82** - Amber terminal on deep navy
+- **Osaka Jade** - Dark teal with jade accents
+- **Kanagawa** - Ink-dark waves and cream ink
+- **Miasma** - Swampy dark greens and ochre
+- **Lumon** - Cold teal, for the severed floor
+- **Ristretto** - Coffee-dark with warm peach
+- **Gruvbox** - Retro groove, warm dark
+- **Everforest** - Cozy green-toned dark
+
+**Light**
+
+- **Lupine** - Clean light with violet-blue accents (default light theme)
+- **Tokyo Night Day** - Daylight counterpart of the default
+- **Solarized Light** - Precision contrast on warm paper
+- **One Light** - Atom's light theme
+- **Ayu Light** - Bright and airy with orange accents
+- **GitHub Light** - Familiar, clean white
+- **Gruvbox Light** - Warm cream paper
+- **Everforest Light** - Soft green-tinted daylight
+- **Kanagawa Lotus** - Kanagawa's lotus-paper palette
+- **Catppuccin Latte** - Warm light pastels
+- **Rosé Pine** - Soft rose-tinted daylight
+- **Flexoki Light** - High-contrast warm paper
+- **White** - Pure monochrome white
 
 ### Changing Your Theme
 
 1. In the sidebar, open the **Appearance** panel (palette icon, near the bottom)
-2. Pick a theme from the list — each option shows its color swatch, and your active theme is marked with a checkmark
-3. Your choice is saved automatically and synced across devices
+2. Use the **Dark / Light** switch at the top to show the half you want —
+   it opens on the half your current theme lives in
+3. Pick a theme from the list — each option shows its color swatch, and your
+   active theme is marked with a checkmark
+4. Your choice is saved automatically and synced across devices
 
 On small screens, open the sidebar with the menu button in the top bar first.