[Dashboard][Phase 1] Build responsive application shell

[rd-code-23]

Summary

• add the shared responsive application shell with a desktop sidebar and mobile header/bottom navigation
• add real Dashboard, Routines, History, and Settings destinations with router-owned active states
• update semantic dark surfaces and the GoForLift red accent
• add accessible navigation tests and preserve the guest-to-dashboard flow
• document readable Tailwind class grouping and clarify the shared cn helper

Scope

This PR implements only the application shell and navigation foundation. Dashboard empty states, identity/logout behavior, and workout data remain in later Dashboard milestone issues.

Testing

• pnpm --filter @goforlift/web test (14 tests passed)
• pnpm --filter @goforlift/web lint
• pnpm --filter @goforlift/web typecheck
• pnpm --filter @goforlift/web build
• desktop and mobile browser inspection

Closes #38
Refs #45