refactor(web): converge navigation, contrast and dialogs

[RomeoCavazza]

What changed

• L01: exposes semantic desktop and mobile navigation with shared active-route resolution
• L02: honors reduced motion, forced colors and platform contrast contracts
• L03: establishes the shared Radix dialog shell and migrates confirmations plus Create Incident
• L04: migrates Create Team and Join Team with fresh state, focus restoration and announced errors

Product boundaries

• keeps the existing OpsWarden visual identity
• does not install Primer or another UI framework
• does not change APIs, PostgreSQL schemas or RBAC
• keeps each accepted lot as an independent commit

Validation

• 81 Vitest tests
• Next.js production build
• ESLint, Prettier and TypeScript
• Cargo workspace check
• focused Playwright suites for navigation, platform contrast, Incident dialogs and Team dialogs
• real Manager Team creation and Responder invitation flow
• deterministic demo reset to 3 Teams, 9 memberships and 20 incidents
• desktop and 320 px captures reviewed for L01-L04