Redesign main app interface

[macaddy2]

Summary

• refreshes the main Fixars app with a calmer waitlist-aligned visual system
• updates shared cards, buttons, inputs, modal chrome, app shell, dashboard, sub-app pages, and responsive rules
• fixes the legacy public Header runtime bug caused by missing isSupabaseConfigured import

Validation

• npm run build passes
• targeted ESLint passes for changed JS files
• full npm run lint still fails on pre-existing repository/design-handoff lint debt
• browser QA: public home, login, dashboard, apps, feed, wallet, profile, settings, ConceptNexus, vestDen, CollaBoard, SkillsCanvas render without framework overlay
• browser QA: mock login works, search overlay works, notification dropdown works, Submit Idea modal fills/submits and updates local UI, mobile dashboard has no horizontal overflow

Scope

• root main app only
• no landing-page/waitlist changes