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/SessionRail.svelte

@@ -39,7 +39,7 @@
 <aside class="flex h-full w-56 shrink-0 flex-col gap-2 overflow-y-auto border-r bg-card/50 p-2">
   <Button variant="outline" size="sm" class="justify-start gap-2" onclick={() => newChat()}>
     <PlusIcon class="size-3.5" />
-    New chat
+    New Task
   </Button>
   <ScrollArea class="min-h-0 flex-1">
     <div class="flex flex-col gap-1 pr-2">