Core UI/UX modernization

[Ryder-MHumble]

Summary

• Modernize core NodeBB UI shell with ZGCI light theme tokens and improved shared components
• Add tracked Harmony and composer template overrides for topic detail, quick reply, composer, and account management
• Preserve runtime override syncing through postinstall to avoid node_modules-only drift

Verification

• npm ci
• node scripts/apply-local-overrides.js
• Benchpress precompile for changed templates
• Targeted ESLint for changed JS
• SCSS compile using NodeBB-style import order

Notes

• Full ./nodebb build was not completed locally because this checkout has no config.json and launches the setup web installer.