feat(ui/graph): optimize graph dynamics, immersive reader, and code blocks

- Fix #16: Implement D3.js transitions and active state badges for Knowledge Graph.
- Fix #12: Implement Immersive Reader Layout with Merriweather typography.
- Fix #20: Professional code block styling with high contrast and monospace fonts.
- Resolve DI runtime error in WASM by adding dummy services.
- Replace generic 'Not Found' message with Nexus preloader.
- Enforce 'no async void' architecture across UI services.

.agent/skills/nexus-ui-engine/SKILL.md

@@ -22,7 +22,7 @@ description: Design System & Component rules for Blazor
     - Light Mode: `--nexus-bg` (`#f8f9fa`), `--nexus-card` (`#ffffff`).
   - **Typography:**
     - UI Elements: `Inter` (Sans-Serif) for controls, menus, and labels.
-    - Reading Content: `Merriweather` (Serif) for books and articles to ensure high readability.
+    - Reading Content: `Merriweather` (Serif) with `line-height: 1.65` and `letter-spacing: -0.01em` for high readability.
   - **Effects:**
     - Subtle neon glows (`box-shadow: 0 0 15px rgba(0, 255, 153, 0.3)`).
     - Glassmorphism for overlays and modals.
@@ -30,6 +30,11 @@ description: Design System & Component rules for Blazor
 - **Adaptive Layouts:**
   - Support `.platform-mobile` and `.platform-desktop` context classes.
   - Handle safe-area insets (`--safe-area-inset-*`) for mobile devices.
+  - **Immersive Reader (Zen Mode):**
+    - Centered content flow: `max-width: 800px`, `margin: 0 auto`.
+    - Paper-white background: `#F9F9F9` for light mode reader canvas.
+    - Dedicated Scrollbars: Custom styled, thin scrollbars with `--nexus-neon` accents.
+    - Reachability: Large `padding-bottom` (e.g., `15rem`) to ensure comfortable reading of end-of-page content.
 
 - **Accessibility (A11y):**
   - Touch Targets: Min `44x44px` on mobile (enforced via CSS variables).