Complete design tokens, add AppShell primary nav, and Button/Card primitives (#1313, #1314, #1315, #1316) - #1459

Summary

• [Design System] Recreate design tokens stylesheet #1313: styles/tokens.css already exists in full (117 lines, dark+light palettes) — nothing was deleted, so there was nothing to "recreate". The real gap: three CSS custom properties were referenced elsewhere in the app only via var(--x, <hardcoded fallback>) (--danger in PlaceBetForm.css, --border-color in OutcomeList.css, --shadow-lg in two account/newsletter pages), bypassing the centralized token system entirely — meaning dark-mode toggling (Event-Driven Frontend Indexing Support #22-23) and contrast-check tooling (Referral System & Affiliate Incentives #21) had nothing to hook into for them. Added --danger/--border-color as aliases of their equivalent existing tokens, a real --shadow-lg value, and the shared @keyframes spin used by the new Button below.
• [Design System] Build app shell layout (header, nav, footer) #1314: layout.tsx already wraps the entire app today, not just the landing page — but there genuinely was no primary navigation anywhere outside the admin section's own local sub-nav. Added AppShell.tsx (header + primary nav across Markets/Statistics/Create Market, plus a conditional Admin link once an admin session exists, + a minimal footer) and wired it into layout.tsx. It intentionally skips / and /admin/*, since LandingPage.tsx and admin/layout.tsx already each own their own full header/nav — avoiding a duplicate header in both places.
• [Design System] Build Button primitive #1315: added components/ui/Button.tsx — consistent variant/loading/disabled handling, generalizing the existing admin-only Button in components/admin/Form.tsx for use outside the admin section (bet placement, market creation, resolution).
• [Design System] Build Card primitive #1316: added components/ui/Card.tsx (Card, CardHeader, CardTitle, CardBody, CardFooter) — a single shared container primitive for market list items (Email Service Integration #57), statistics tiles (Analytics Event Tracking API #49), and admin panels (Create Component Library Documentation with Storybook #89-97) to converge on.

No existing consumers were migrated to Button/Card — out of scope for this PR, which builds the primitives the issues asked for.

Test plan

• Verified brace/paren balance on all new/changed files via static analysis (no build/test/install commands run)
• Traced every var(--x) usage across the frontend against tokens.css's defined custom properties to find the real [Design System] Recreate design tokens stylesheet #1313 gap
• Traced nav/header presence across app/page.tsx, app/markets/page.tsx, app/statistics/page.tsx, and app/admin/layout.tsx to confirm the real [Design System] Build app shell layout (header, nav, footer) #1314 gap and avoid duplicating existing chrome

Closes #1313
Closes #1314
Closes #1315
Closes #1316