feat: add styled error pages, fix type errors, add svelte-check CI

[rubenhensen]

Summary

Closes #42

• Custom error page (src/routes/+error.svelte): Card-based layout showing HTTP status code, contextual title/message (404 vs server errors), and a home link. Styled with design tokens, supports dark mode.
• i18n translations: Added error section to both en.json and nl.json with Dutch and English messages.
• Fixed all 14 pre-existing type errors and 2 warnings found by svelte-check:
  • vite.config.ts: Removed unused path import/__dirname, simplified server.fs.allow to relative '..', cast esbuild polyfill plugins with as any to fix type mismatch
  • FileInput.svelte: Replaced non-existent Dropzone.autoDiscover with Dropzone.discover = () => []
  • +page.svelte: Renamed Error import to ErrorPanel to avoid shadowing the built-in Error constructor
  • debug/before-send/+page.svelte, debug/done/+page.svelte, debug/upload/+page.svelte: Added missing serverError: false to EncryptState objects
  • download/+page.svelte: Cast selectedLang to 'nl' | 'en' to match YiviCore language option type
  • Decrypt.svelte: Added @type {any} JSDoc to session result.url destructured parameter to fix SessionMappings type mismatch
  • Chip.svelte: Changed const isInteractive to $derived to fix state_referenced_locally warning
  • EncryptionProgress.svelte: Changed const to to $derived to fix state_referenced_locally warning
• CI: Added svelte-check job to .github/workflows/ci.yml — runs on every push/PR, fails on warnings or errors (Node 24, actions/setup-node@v6)
• Added Svelte MCP server config (.mcp.json) for development tooling

Test plan

• Visit a non-existent route (e.g. /nonexistent) and verify the styled 404 page appears
• Toggle dark mode and verify the error page respects the theme
• Switch language to Dutch and verify translated error messages
• Verify npx svelte-check --threshold warning passes with 0 errors and 0 warnings
• Verify npm run build succeeds
• Verify the new CI svelte-check job passes on the PR