Improve Mermaid diagram styling - smaller, more compact nodes

- Add Mermaid init config for smaller 12px font and cleaner theme
- Use rounded corners (rx:5, ry:5) for all node types
- Add subtle colored borders matching node type colors
- Thinner 1px stroke width for cleaner appearance

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app.py

@@ -230,7 +230,10 @@ def generate_mermaid_from_graph(graph: LineageGraph) -> str:
     if not graph.nodes:
         return "graph TD\n    A[No data to display]"
 
-    lines = ["graph TD"]
+    lines = [
+        "%%{init: {'theme': 'base', 'themeVariables': {'fontSize': '12px', 'fontFamily': 'arial', 'primaryColor': '#e8f5e9', 'primaryBorderColor': '#4caf50', 'lineColor': '#666'}}}%%",
+        "graph TD"
+    ]
 
     # Group nodes by category for subgraphs
     categories = {}
@@ -240,18 +243,18 @@ def generate_mermaid_from_graph(graph: LineageGraph) -> str:
             categories[cat] = []
         categories[cat].append(node)
 
-    # Generate nodes with styling
+    # Generate nodes with styling - compact with rounded corners and subtle borders
     node_styles = {
-        "source": "fill:#e1f5fe",
-        "external_api": "fill:#e1f5fe",
-        "table": "fill:#e8f5e9",
-        "view": "fill:#f3e5f5",
-        "model": "fill:#fff3e0",
-        "report": "fill:#fce4ec",
-        "dimension": "fill:#e0f7fa",
-        "fact": "fill:#fff8e1",
-        "destination": "fill:#ffebee",
-        "task": "fill:#f5f5f5"
+        "source": "fill:#e3f2fd,stroke:#1976d2,stroke-width:1px,rx:5,ry:5",
+        "external_api": "fill:#e3f2fd,stroke:#1976d2,stroke-width:1px,rx:5,ry:5",
+        "table": "fill:#e8f5e9,stroke:#388e3c,stroke-width:1px,rx:5,ry:5",
+        "view": "fill:#f3e5f5,stroke:#7b1fa2,stroke-width:1px,rx:5,ry:5",
+        "model": "fill:#fff3e0,stroke:#f57c00,stroke-width:1px,rx:5,ry:5",
+        "report": "fill:#fce4ec,stroke:#c2185b,stroke-width:1px,rx:5,ry:5",
+        "dimension": "fill:#e0f7fa,stroke:#0097a7,stroke-width:1px,rx:5,ry:5",
+        "fact": "fill:#fff8e1,stroke:#ffa000,stroke-width:1px,rx:5,ry:5",
+        "destination": "fill:#ffebee,stroke:#d32f2f,stroke-width:1px,rx:5,ry:5",
+        "task": "fill:#fafafa,stroke:#616161,stroke-width:1px,rx:5,ry:5"
     }
 
     # Add subgraphs for categories