Add Forum to the app sidebar under Leaderboard for everyone #31

Outcome

Every signed-in reader sees a Forum entry in the navigation sidebar, placed directly under Leaderboard.

Work

• Add the sidebar section in the shared sidebar sections plug/component used by the application shell.
• Link to /forum; active state follows the current path like the other entries.
• Confirm ordering: Leaderboard, then Forum.
• Pixel proof of the sidebar in light and dark palettes.

Acceptance criteria

• The link appears for every authenticated user on every page that renders the shell.
• No layout shift when navigating between Forum and other surfaces.

Verification

Sidebar state tests updated; pixel proof; mix precommit.