chore: roll back major dependency updates

[austinlparker]

Summary

• Rolls back Next.js from v15 to v14.2.28
• Rolls back React from v19 to v18.2.0
• Rolls back Tailwind CSS from v4 to v3.4.1
• Updates visual tests to work with redesigned components
• Updates Renovate config to disable major version updates for these packages
• Adds missing UI component dependencies

Test plan

• ✅ Build successfully runs with

[email protected] build
next build

▲ Next.js 14.2.28

Creating an optimized production build ...
✓ Compiled successfully
Skipping linting
Checking validity of types ...
Collecting page data ...
Generating static pages (0/8) ...
Generating static pages (2/8)
Generating static pages (4/8)
Generating static pages (6/8)
✓ Generating static pages (8/8)
Finalizing page optimization ...
Collecting build traces ...

Route (app) Size First Load JS
┌ ○ / 47.4 kB 135 kB
├ ○ /_not-found 875 B 88 kB
├ ƒ /api/revalidate 0 B 0 B
├ ƒ /api/test 0 B 0 B
├ ƒ /api/webhook 0 B 0 B
└ ƒ /feed 0 B 0 B

• First Load JS shared by all 87.1 kB
  ├ chunks/117-7cb1057c95193d92.js 31.6 kB
  ├ chunks/fd9d1056-78c98a0c3058eafb.js 53.7 kB
  └ other shared chunks (total) 1.85 kB

○ (Static) prerendered as static content
ƒ (Dynamic) server-rendered on demand

• ✅ All Playwright tests pass, including visual regression tests
• ✅ App functions as expected in development mode

Closes #50

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