feat: design revamp — fluid layout, full Tailwind removal, first-launch seed

[slverma]

Summary

• Fluid layout & tab strip: introduces --rl-* CSS custom properties for
  fluid font/spacing sizing across the request editor; tab strip gains
  scroll with fade-edge indicators; response section and key-value rows are
  fluid-sized.
• Full Tailwind removal: replaces all Tailwind utility strings with
  semantic CSS classes across all three webview bundles (sidebar sidebar.css,
  editor editor/styles.css, request request/styles.css); remaining
  title="" tooltip attributes migrated to <Tooltip> component.
• New icon components: adds CollectionAddIcon and FolderImportIcon
  SVG components; integrates them into Sidebar and ImportDropdown.
• First-launch seed data: seeds an example JSONPlaceholder collection
  into globalState on first install; rolls back cleanly on partial failure.
• Packaging & publishing: adds Open VSX Registry publishing to the
  release pipeline (packages once to .vsix, publishes to both registries);
  fixes .vscodeignore to exclude source/config files; sets noExternal: true
  to bundle all extension deps.

Test Plan

• Open REST Lab fresh — JSONPlaceholder seed collection appears in sidebar
• Verify sidebar renders without Tailwind classes in DevTools (no bg-, px-, etc.)
• Verify editor (Environments, Settings) and request tabs (Auth, Headers, Params, Body, Cookies) render correctly
• Tab strip scrolls horizontally when tabs overflow; fade edges appear/disappear
• Tooltips appear on all icon buttons (previously used title="")
• Run vsce package locally — confirm src/, docs/, scripts/ are excluded from the .vsix
• Confirm extension activates without runtime errors after npm run build