feat(theme): four themes with an actual hue — Indigo, Ocean, Plum, Nordic

The seven existing themes are warm beige, cool grey, sage grey, slate, warm
dark, graphite and black — every one neutral, six of the seven accented orange.
Picking a theme changed the shade of grey and little else.

These colour the SURFACES, not just the accent, and each takes a different
primary so the picker tells them apart at a glance: violet on deep indigo, cyan
on deep teal, magenta on aubergine, indigo on a crisp cool white.

The semantic colours stay recognisable as themselves. A logger is read for
hours and "red means a problem" cannot become a decorative choice, so the hue
budget went on the surfaces and the primary. Where a theme's primary would have
collided with a meaning, the MEANING kept its identity and the ornament moved:
Ocean's info is blue rather than cyan, Plum's danger is red rather than rose,
and the matrix entity ramp shifts to cyan under Indigo and teal under Nordic so
a "confirmed" cell never reads as a button.

Everything else follows for free: the grids resolve var(--…) at runtime, so
they re-skin with no re-render. The three shared registrations that do NOT
follow automatically are done — the chart palette (light vs dark ramp) and the
date-picker icon inversion, which is keyed on an explicit list of dark themes.

frontend/src/lib/theme.tsx

@@ -6,11 +6,16 @@ import { GetUIPref } from '../../wailsjs/go/main/App';
 // CSS variables in style.css key off of. 'auto' follows the OS light/dark
 // preference. The choice is persisted (localStorage + portable UI pref, so it
 // travels with the data/ folder like the language).
-export type ThemeChoice = 'auto' | 'light-warm' | 'light-cool' | 'light-sage' | 'dim-slate' | 'dark-warm' | 'dark-graphite' | 'high-contrast';
+export type ThemeChoice = 'auto' | 'light-warm' | 'light-cool' | 'light-sage' | 'light-nordic'
+  | 'dim-slate' | 'dark-warm' | 'dark-graphite' | 'dark-indigo' | 'dark-teal' | 'dark-plum' | 'high-contrast';
 
-// Selectable, concrete themes (excludes 'auto') in display order.
+// Selectable, concrete themes (excludes 'auto') in display order: lights first,
+// then darks, with high-contrast last — it is an accessibility choice, not a
+// taste one, and listing it among the moods buries it.
 export const CONCRETE_THEMES: Exclude<ThemeChoice, 'auto'>[] = [
-  'light-warm', 'light-cool', 'light-sage', 'dim-slate', 'dark-warm', 'dark-graphite', 'high-contrast',
+  'light-warm', 'light-cool', 'light-sage', 'light-nordic',
+  'dim-slate', 'dark-warm', 'dark-graphite', 'dark-indigo', 'dark-teal', 'dark-plum',
+  'high-contrast',
 ];
 
 export const LS_KEY = 'opslog.theme';