Add multi-accent palettes and a brighter Light Green theme (ticket 048)

Every built-in theme now exposes three accents (--c-accent/-2/-3) instead
of one, so PanelChart's series palette draws distinct colours per theme
via color-mix() rather than a fixed, theme-independent hex list. Existing
presets keep their current base surface/border/text/accent look — the two
new accent vars are purely additive. Adds a new "Light Green" scheme
(light-surfaced, same green/magenta hue family as "Green") selectable
alongside the existing built-ins, both in ColorScheme (frontend + backend)
and the settings picker.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/lib/useSettingsPanel.ts

@@ -21,7 +21,17 @@ import {
 const INJECTED_THEME_ATTR = 'data-ottyl-theme-css-id'
 
 // Themes selectable independently of `system`, each backed by a `html.<name>` block in `index.css`.
-const THEME_CLASSES = ['light', 'dark', 'red', 'green', 'blue', 'yellow', 'pink', 'orange'] as const
+const THEME_CLASSES = [
+  'light',
+  'dark',
+  'red',
+  'green',
+  'blue',
+  'yellow',
+  'pink',
+  'orange',
+  'light_green',
+] as const
 
 // Reflects `colorScheme` onto `<html>` as a class + `data-color-scheme`; when a custom theme is active, no built-in class is added at all, since a class selector would out-specificity its injected `:root` override (see `applyCustomThemeCss`) and mask it.
 function applyColorScheme(colorScheme: ColorScheme, customThemeActive: boolean) {