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/components/SettingsModal.tsx

@@ -403,9 +403,13 @@ const THEME_SWATCH: Record<Exclude<ThemeChoice, 'auto'>, { bg: string; card: str
   'light-warm':    { bg: '#e8dfc9', card: '#faf6ea', accent: '#b8410c' },
   'light-cool':    { bg: '#f4f6f8', card: '#ffffff', accent: '#2563eb' },
   'light-sage':    { bg: '#eef1ec', card: '#f8faf6', accent: '#2f855a' },
+  'light-nordic':  { bg: '#eef1f7', card: '#ffffff', accent: '#4f46e5' },
   'dim-slate':     { bg: '#343b47', card: '#3d4552', accent: '#fb923c' },
   'dark-warm':     { bg: '#221d18', card: '#2e2820', accent: '#e07a2e' },
   'dark-graphite': { bg: '#16181d', card: '#1f232b', accent: '#f97316' },
+  'dark-indigo':   { bg: '#0e0f1f', card: '#181a30', accent: '#7c6cff' },
+  'dark-teal':     { bg: '#061c21', card: '#0d2c33', accent: '#22d3ee' },
+  'dark-plum':     { bg: '#180f1e', card: '#251830', accent: '#f472b6' },
   'high-contrast': { bg: '#000000', card: '#0d0d0d', accent: '#ff7a1a' },
 };