fix(mobile): ensure header stays fixed across all pages (#52)

* Initial plan

* fix(mobile): ensure header stays fixed on all pages

Add !important to position: sticky and top: 0 in mobile breakpoint
to prevent any CSS specificity issues that could cause the header
to lose its fixed positioning when navigating between chapters.

docs(agent): mandate conventional commits in instructions

Co-authored-by: aih <217356+aih@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: aih <217356+aih@users.noreply.github.com>

Author: Copilot

.github/copilot-instructions.md

@@ -109,7 +109,11 @@ The application is deployed on Vercel. Configuration is in `CompendiumUI/vercel.
 - **CSS**: USWDS design system patterns
 - **Python**: Standard Python conventions, no linter configured
 - **Comments**: Minimal; code should be self-explanatory
-- **Commit Messages**: Use conventional commit messages
+- **Commit Messages**: **MUST** use [Conventional Commits](https://www.conventionalcommits.org/) format for all commits:
+  - Format: `<type>(<scope>): <subject>`
+  - Types: `feat`, `fix`, `docs`, `style`, `refactor`, `test`, `chore`
+  - Example: `fix(mobile): ensure header stays fixed on scroll`
+  - Example: `docs: update README with new installation steps`
 - **File Naming**: 
   - XHTML chapters: `ch{number}-{title}.html` (e.g., `ch200-copyrightability.html`)
   - Python scripts: lowercase with underscores

CompendiumUI/style.css

@@ -1075,8 +1075,8 @@ button:focus,
 	.header {
 	    grid-column: 1 / -1;
 	    grid-row: 1;
-	    position: sticky; /* Make header sticky on mobile */
-	    top: 0; /* Stick to top of viewport */
+	    position: sticky !important; /* Make header sticky on mobile - important to prevent override */
+	    top: 0 !important; /* Stick to top of viewport */
 	    z-index: 100; /* Ensure header stays above other content */
 	}