feat(examples): modernize navigation, landing, and theme pages

[axis-d0op]

Summary

UI/UX pass on the examples app, focused on the side navigation, landing page, and theme token page, plus a unified copy button.

Side navigation

• Rebuilt the sidebar as a custom component (no more NavDrawer) so icons and labels align consistently.
• Active item highlight (brand tint + filled icon + left accent bar), hover/pressed states, collapsible Components/Styles groups with an animated chevron and a sub-item guide line.
• Slimmer width and a lighter panel surface; unified the far-left icon rail onto the same surface (no more black stripe).

Top bar

• Removed the clipped placeholder controls ("Use application menu", "Drawer version", "I AM CENTER") and moved the demo toggles into the help menu.
• Defaults to the labeled application.

Landing page

• Brand kicker pill, tightened hero hierarchy, and cleaner cards (icon badge instead of a rotating watermark).

Theme token page

• Search now filters all sections (custom color, custom utility, and Fluent tokens), matching on token name and both values.
• Matched substrings are highlighted, each section shows a live count, and there's an empty state plus a subtle row entrance animation.
• Rebuilt the token list as a readable table with swatch chips.

Shared copy button

• Added examples/src/components/copy-button.tsx with consistent styling and a "Copied!" checkmark confirmation.
• Reused it for token, icon, illustration, and code-block copy actions (previously three separate implementations with an inconsistent icon and no feedback).

Verification

• pnpm format, pnpm --filter examples lint (tsc + Biome), pnpm test, and pnpm build all pass.
• Verified interactively in the dev server.

[markuslindmark]

allright