feat: Add static changelog page

Adds a new static page accessible at /changelog to display site updates and changes manually.

- Creates a new Flask Blueprint in `routes/changelog.py` to handle the route logic.
- Registers the `changelog_bp` blueprint in `app.py`.
- Creates the corresponding template `templates/changelog/changelog.html` extending the base layout.
- Adds a link to the Changelog page in the main sidebar navigation in `templates/base.html`, using an archive icon for consistency.
- Applies basic card styling to the changelog page content for improved visual structure.

routes/changelog.py

@@ -0,0 +1,15 @@
+from flask import Blueprint, render_template, current_app
+from flask_htmx import htmx
+from jinja2_fragments import render_block
+
+changelog_bp = Blueprint('changelog', __name__)
+
+@changelog_bp.route('/')
+def show_changelog():
+    """Renders the changelog page."""
+    template_name = 'changelog/changelog.html' # Path relative to templates/
+    if htmx:
+        # If request is from HTMX, render only the content block
+        return render_block(current_app.jinja_env, template_name, 'content')
+    # Otherwise, render the full page
+    return render_template(template_name)