feat: Implement Sprint 8 - Portfolio website expansion (MVP)

New pages:
- Home: Redesigned with hero, impact stats, featured project
- About: 6-section professional narrative
- Projects: Hub with 4 project cards and status badges
- Resume: Inline display with download placeholders
- Contact: Form UI (disabled) with contact info
- Blog: Markdown-based system with frontmatter support

Infrastructure:
- Blog system with markdown loader (python-frontmatter, markdown, pygments)
- Sidebar callback for active state highlighting on navigation
- Separated navigation into main pages and projects/dashboards groups

Closes #36, #37, #38, #39, #40, #41, #42, #43

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

portfolio_app/callbacks/__init__.py

@@ -1,5 +1,5 @@
 """Application-level callbacks for the portfolio app."""
 
-from . import theme
+from . import sidebar, theme
 
-__all__ = ["theme"]
+__all__ = ["sidebar", "theme"]

portfolio_app/callbacks/sidebar.py

@@ -0,0 +1,25 @@
+"""Sidebar navigation callbacks for active state updates."""
+
+from typing import Any
+
+from dash import Input, Output, callback
+
+from portfolio_app.components.sidebar import create_sidebar_content
+
+
+@callback(  # type: ignore[misc]
+    Output("floating-sidebar", "children"),
+    Input("url", "pathname"),
+    prevent_initial_call=False,
+)
+def update_sidebar_active_state(pathname: str) -> list[Any]:
+    """Update sidebar to highlight the current page.
+
+    Args:
+        pathname: Current URL pathname from dcc.Location.
+
+    Returns:
+        Updated sidebar content with correct active state.
+    """
+    current_path = pathname or "/"
+    return create_sidebar_content(current_path=current_path)