feat: add testimonials marquee to landing page

[starc007]

What

Testimonials section on the landing page (between the components grid and the Contact CTA).

• Custom card (`testimonial-card.tsx`) in beUI's own style: avatar, name + blue verified seal, @handle, tweet text with mentions/hashtags/links in the accent color and trailing media links stripped. Whole card links to the tweet.
• Data via `react-tweet`: `getTweet` + `enrichTweet` on the server (IDs only in code). Rendered statically — no async component streams to the client, avoiding the React 19 / Next 15 Flight bug (`enqueueModel is not a function`). Failed/deleted tweets drop out; section returns null if all fail.
• Two-row marquee scrolling opposite directions using beUI's own `Marquee`, compact fixed-width cards, edge fade, pause-on-hover.

Also

• Fix `Marquee` seam gap: the outer wrapper had no gap, so the loop point (last card → first card of the duplicated track) butted together while in-track items were spaced. Added matching gap to the wrapper. Affects the shipped `marquee` registry component too.
• Adds `react-tweet` dependency.

Test plan

• `bun run check` clean (typecheck, lint, registry — 41 components)