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/components/ui/sonner/sonner.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 	import { Toaster as Sonner, type ToasterProps as SonnerProps } from "svelte-sonner";
-	import { mode } from "mode-watcher";
+	import { getTheme } from "$lib/stores/theme.svelte";
 	import Loader2Icon from '@lucide/svelte/icons/loader-2';
 	import CircleCheckIcon from '@lucide/svelte/icons/circle-check';
 	import OctagonXIcon from '@lucide/svelte/icons/octagon-x';
@@ -11,7 +11,7 @@
 </script>
 
 <Sonner
-	theme={mode.current}
+	theme={getTheme() === 'light' ? 'light' : 'dark'}
 	class="toaster group"
 	style="--normal-bg: var(--color-popover); --normal-text: var(--color-popover-foreground); --normal-border: var(--color-border);"
 	{...restProps}