fix: keep sidebar visible in Firefox

[chethanuk]

Summary

Changes

Testing

• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace passes
• Live integration tested (if applicable)

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries

[jaberjaber23]

Confirmed this is the correct fix for #1036 — Firefox's <template x-if> handling of Alpine transitions is flaky, and the x-show + x-cloak + x-transition replacement is the idiomatic Alpine workaround. The added [x-cloak] { display: none !important; } in layout.css is needed to prevent FOUC before Alpine boots.

CI rollup is empty on this branch; please rebase on latest main (we just merged #1041) to get checks running. Assuming they go green, this is ready to land.