feat: add theme system, fonts, graph styling, rename Overview→Tasks

- Terracotta (light) and Carbon (dark) themes with toggle
- Inknut Antiqua headings, DM Sans body
- Dot grid background on EntityGraph and GraphBackground
- Theme-adaptive graph colors on EntityGraph
- Art Nouveau chat styling (borders, underlines, blockquote quotes)
- Bullet point styles in chat prose
- Task goal in header, rename Overview→Tasks, New Task labels
- Logo uses var(--primary) for theme awareness

web/src/lib/stores/theme.svelte.ts

@@ -0,0 +1,46 @@
+export type Theme = 'light' | 'dark'
+
+const STORAGE_KEY = 'oikos-theme'
+
+function applyClass(theme: Theme): void {
+  const root = document.documentElement
+  if (theme === 'dark') {
+    root.classList.add('dark')
+  } else {
+    root.classList.remove('dark')
+  }
+}
+
+function storedTheme(): Theme {
+  if (typeof localStorage === 'undefined') return 'dark'
+  const v = localStorage.getItem(STORAGE_KEY)
+  if (v === 'light' || v === 'dark') return v
+  return window.matchMedia('(prefers-color-scheme: light)').matches ? 'light' : 'dark'
+}
+
+let current: Theme = $state(storedTheme())
+
+applyClass(current)
+
+export function setTheme(t: Theme): void {
+  current = t
+  applyClass(t)
+  if (typeof localStorage !== 'undefined') {
+    localStorage.setItem(STORAGE_KEY, t)
+  }
+}
+
+export function getTheme(): Theme {
+  return current
+}
+
+export function toggleTheme(): Theme {
+  const next = current === 'dark' ? 'light' : 'dark'
+  setTheme(next)
+  return next
+}
+
+export const THEME_LABELS: Record<Theme, string> = {
+  light: 'Terracotta',
+  dark: 'Carbon'
+}