fix(frontend): only request platform icons that ship

[gantoine]

Description

Fixes #4671

The v2 platform icon components tried /assets/platforms/<slug>.svg, then .ico, and let the browser 404 on every miss before landing on default.ico. The prefetch cache did the same probing for every platform on app load. That's where the ps.svg, win.svg and ps.ico 404s in the issue come from.

This takes the approach from #4728 by @andest01 (look icons up in a list of the files that ship instead of probing) and narrows it to the fix:

• Build-time manifest. A small Vite plugin (scripts/platformIconManifest.ts) exposes the icons under frontend/assets/platforms/ as virtual:platform-icons, a slug to filename map (.svg wins over .ico). It reloads when an icon is added or removed under the dev server. src/v2/utils/platformIcons.ts exports platformIconUrl(slug, fsSlug); unknown slugs go straight to default.ico without a request.
• Why not import.meta.glob. An eager ?url glob over the folder would import every icon just to read its name. I measured it: it emits 233 hashed copies of the icons (~5.6 MB) into dist/assets and inlines 26 small ones as data: URIs into the lib chunk (165 KB to 273 KB). The virtual module only adds the names.
• Prefetch. prefetchPlatformIcons skips slugs that ship nothing and fetches one URL per shipped slug.
• RPlatformIcon becomes components/shared/PlatformIcon. It now depends on platform-specific asset knowledge, so it no longer qualifies as a lib/ primitive. Consumers import it by path; the story moved with it, and the Storybook test harness (test/storybook.test.ts) now covers components/shared stories too, so it keeps rendering and running its axe checks in CI. CachedPlatformIcon uses the same resolver.

Behavior change: custom icon mounts. The docs let users bind-mount their own icons over /var/www/html/assets/platforms. The new UI now only knows the icons built into the image, so it ignores those mounts. This is intended since the classic UI is going away; rommapp/docs#150 removes the custom icon guide and should merge when this ships.

Left out on purpose: the Storybook staticDirs/fixtures work from #4728 (separate PR), and merging CachedPlatformIcon into PlatformIcon (follow-up).

flowchart LR
  A["assets/platforms/*.svg|ico"] -->|"build: readdir"| M["virtual:platform-icons"]
  M --> R["platformIconUrl(slug, fsSlug)"]
  R -->|shipped| U["/assets/platforms/file"]
  R -->|not shipped| D["default.ico, no request"]
  R --> P["PlatformIcon / CachedPlatformIcon"]
  M --> F["prefetchPlatformIcons: one fetch per shipped slug"]

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AI assistance: This PR was written with Claude Code (Claude Opus 5.5): the implementation, tests, review passes and this description. I reviewed the approach against #4728 and the diff before opening.

Checklist

• I've tested the changes locally
• I've updated relevant comments
• I've assigned reviewers for this PR
• I've added unit tests that cover the changes

Local checks: npm run typecheck, typecheck:scripts, npm run test (235 files, 2655 tests; the Storybook harness now runs 408 stories including 31 shared ones), npm run build, storybook build, and trunk check. I did not do a manual browser pass: nothing visual changes, only which URL each icon requests.

Screenshots (if applicable)

No visual change.

🤖 Generated with Claude Code

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[greptile-apps]

Confidence Score: 4/5

[Medium risk] Refactors platform icon loading to skip unavailable icons.

This PR should not merge until documented runtime-mounted custom platform icons remain usable and the repository-rule violations are resolved.

Summary

The PR generates a build-time platform-icon manifest, selects known icon URLs without probing missing files, updates prefetching, and moves the v2 icon component out of the primitive library.

• The fixed manifest conflicts with the documented runtime custom-icon workflow.
• Icon-load errors no longer try other available platform-specific candidates.
• Development refresh and two repository test/comment requirements need attention.

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