fix: via.placeholder.com shut down in 2024 (now connection-refused)

[kiluazen]

apps/test/app/static/page.tsx references via.placeholder.com/*. The service was shut down in 2024 and the DNS record no longer resolves.

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Why this is needed

via.placeholder.com was shut down in 2024 and its DNS record no longer resolves — every request now hangs or returns a connection error. Any example that references it renders a broken-image icon (and any code that depends on a 200 from it silently breaks).

Verify in any shell:

curl -sI --max-time 3 https://via.placeholder.com/300x200 || echo "connection refused / DNS gone"

What this PR changes

Fixes the Markdown image link inside the streamdown static-rendering test page so the Images section renders an actual image when the Streamdown docs/demo are served.

Replacement details

Docs-test-only change. placehold.co/300x200 uses the same /<W>x<H> path shape and returns a 200 PNG.

Background

I'm tracking dead image-placeholder endpoints (source.unsplash.com/*, via.placeholder.com/*, placekitten.com/*) across public repos as part of tteg, a tiny CLI/HTTP API I built so projects can drop in real Unsplash photos without registering an Unsplash app or managing API keys. tteg is not a dependency of this PR — the diff uses the dependency-free canonical replacement (placehold.co, which accepts the same path shape as via.placeholder.com). If you want topic-matched real photos as a follow-up, the no-key HTTP API is at https://tteg-api-53227342417.asia-south1.run.app/search?q=<query>&n=1 (CORS-on, no auth).

One extra artifact you may find handy: a public scanner at https://tteg.kushalsm.com/scan?url= that highlights dead-placeholder patterns in any landing page — useful for verifying the fix lands and for finding other places dead URLs slipped in. Source: https://github.com/kiluazen/tteg-landing/blob/main/scan.html.

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