fix(docs): escape parentheses and slashes in Mermaid diagram

Mermaid parser requires special characters in node labels to be
wrapped in double quotes. Fixed:
- Metrics[(Dice)] → Metrics["Metrics (Dice)"]
- UI[Gradio UI / NiiVue] → UI["Gradio UI / NiiVue"]

See: https://mermaid.js.org/syntax/flowchart.html

README.md

@@ -102,8 +102,8 @@ graph TD
     Loader -->|Stage Files| Staging[Staging Dir]
     Staging -->|Mount Volume| Docker[DeepISLES Container]
     Docker -->|Inference| Results[Prediction Mask]
-    Results -->|Load| Metrics[Metrics (Dice)]
-    Results -->|Render| UI[Gradio UI / NiiVue]
+    Results -->|Load| Metrics["Metrics (Dice)"]
+    Results -->|Render| UI["Gradio UI / NiiVue"]
 ```
 
 ## License