feat: desktop-only gate for mobile viewports

[GeneralJerel]

Summary

• Adds a fixed full-screen overlay on viewports < 640px with a "Desktop Only" message
• Uses Tailwind's sm:hidden so it's pure CSS — no JS resize listeners
• Includes a "View on GitHub" link so mobile visitors still have somewhere to go

Closes #69

Test plan

• Resize browser below 640px — overlay appears covering the app
• Resize above 640px — overlay disappears, app is fully usable
• Test on actual mobile device or Chrome DevTools device mode
• "View on GitHub" link opens correctly

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[GeneralJerel]

Self-Review Findings

Verdict: No merge block (P0=0, P1=0)

Clean, minimal 30-line change — fixed overlay on <640px screens with "Desktop Only" message and GitHub link.

ID	Sev	Category	File	Issue	Suggested fix
F1	P2	Accessibility	apps/app/src/app/page.tsx	Overlay div has no role or aria-label — screen readers won't understand this is a blocking modal	Add role="alert" or role="dialog" aria-modal="true" aria-label="Desktop only"
F2	P3	UX	apps/app/src/app/page.tsx	Full app still boots behind the overlay on mobile (agent connection, CopilotKit hooks) — wasted resources	Acceptable for now; could skip agent init on mobile later if perf matters
F3	P3	Style	apps/app/src/app/page.tsx	z-[100] is a high arbitrary z-index; demo gallery drawer uses z-50 — could get fragile with more overlays	Consider CSS variable or constant for z-index layers if more overlays are planned

Open Questions

• Should the GitHub link open the repo root or a specific mobile-related issue?
• Is there a plan to make the app responsive, or is this a permanent gate?

🤖 Generated with Claude Code

[GeneralJerel]

Issues resolved by PR 71, no longer need this