Optimize hub images: pjpg CDN renditions and stale cleanup

[bharvey88]

What does this change?

Closes #5. Shrinks the hub images and stops stale files from lingering.

scripts/fetch_images.py now requests Shopify CDN sources as small
progressive-JPEG renditions (width=600&format=pjpg). I verified empirically
that the store ignores format=webp (returns the original PNG) but
honors format=pjpg (returns image/jpeg), so pjpg is the only conversion
that actually takes effect.

• The six product PNGs drop from 700-900KB each to 60-85KB. images/ goes from
  ~5.2MB to ~840KB.
• Renditions save as .jpg and the previous extension is deleted, so stale
  .png files no longer linger in the repo or the deploy (remove_stale).
• Non-Shopify sources (msr-1's wiki image) are left untouched when the file
  already exists, so its manual resize from Core installer: hub, device wizard, registry, CI #1 is not clobbered by a full-size
  re-download.
• New scripts/test_fetch_images.py (stdlib unittest, no deps) covers the
  rendition URL building and stale cleanup, wired into the registry CI job.
• Regenerated the ten hub images (all valid 600px-wide JPEGs; cast-1 is 600x450).

Heads up: this PR includes the regenerated image binaries, so it is worth a
quick eyeball on image quality before merging.

Checklist

• Playwright tests pass locally (cd tests && npx playwright test) (11 passed)
• Any new runtime file/dir is added to the cp list in .github/workflows/pages.yml (n/a: images/ is already copied; the new files are scripts/tests, not shipped)
• No new build step, framework, or runtime CDN dependency (stdlib only)
• If devices.json changed: python scripts/validate_registry.py passes (only image fields changed; validator ran green)

🤖 Generated with Claude Code

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 77e2b871-df38-44c0-98fc-b83501776bf8

📥 Commits

Reviewing files that changed from the base of the PR and between b45fe28 and 49c8cbb.

⛔ Files ignored due to path filters (15)

• images/air-1.jpg is excluded by !**/*.jpg
• images/air-1.png is excluded by !**/*.png
• images/btn-1.jpg is excluded by !**/*.jpg
• images/cast-1.jpg is excluded by !**/*.jpg
• images/msr-2.jpg is excluded by !**/*.jpg
• images/msr-2.png is excluded by !**/*.png
• images/mtr-1.jpg is excluded by !**/*.jpg
• images/mtr-1.png is excluded by !**/*.png
• images/plt-1.jpg is excluded by !**/*.jpg
• images/plt-1.png is excluded by !**/*.png
• images/pump-1.jpg is excluded by !**/*.jpg
• images/r-pro-1.jpg is excluded by !**/*.jpg
• images/r-pro-1.png is excluded by !**/*.png
• images/temp-1.jpg is excluded by !**/*.jpg
• images/temp-1.png is excluded by !**/*.png

📒 Files selected for processing (4)

• .github/workflows/validate.yml
• devices.json
• scripts/fetch_images.py
• scripts/test_fetch_images.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch perf/5-image-optimization

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