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/SessionGraph.svelte

@@ -343,6 +343,12 @@
         onpointerup={onUp}
         onpointercancel={onUp}
       >
+        <defs>
+          <pattern id="dot-grid" width="12" height="12" patternUnits="userSpaceOnUse">
+            <circle cx="2" cy="2" r="0.8" fill="var(--border)" opacity="0.75" />
+          </pattern>
+        </defs>
+        <rect width={cw} height={ch} fill="url(#dot-grid)" />
         <g>
           {#each links as link}
             {@const s = endpoint(link.source)}