Fix collapsed sidebar toggle

[bradleymackey]

Summary

• replace the collapsed sidebar's hidden-checkbox label with a real accessible button
• keep collapse state in one data-sidebar-collapsed source and persist it to local storage
• update the button label, expanded state, and Lucide icon for both transitions
• cover signed-in and logged-out sidebar rendering plus the generated asset contract

Testing

• go test -count=1 -run 'TestUIShellRendersResponsiveAccessibleSidebar|TestUIShellRemovesCollapsedLabelsFromLoggedOutSidebar|TestUIShellProvidesAccessibleIconTooltips' ./internal/server
• DATABASE_URL='postgres://track:track@localhost:5436/track?sslmode=disable' go test -count=1 -coverprofile=/tmp/track40-server-cover.out ./internal/server
• node --check internal/server/static/app.js

TRACK-40