fix(admin,core): stop sending and tolerate empty image accent

[poboll]

Summary

• the write page builds images entries for images scraped from the markdown body / cover with a hardcoded accent: '' (WriteRouteViewsContent addImageSrc), and ImageSchema.accent (zHexColor.optional()) rejects the empty string — so the whole save request fails with a schema error (reported as innei-dev/yohaku#160, but both ends live in this repo)
• addImageSrc now omits accent instead of defaulting to '' (the field stays optional on the model)
• ImageSchema.accent additionally preprocesses ''/null to undefined, so any other client sending an empty accent no longer fails the entire payload

Notes

• invalid colors (#zzz) are still rejected — validation is not loosened
• verified: full tsc --noEmit on apps/admin (0 errors); zod behavior checked against zod 4.4.3 (empty/null pass as undefined, #e095a4 passes, #zzz rejected)
• the exported openapi.json is byte-identical before/after the schema change, so the OpenAPI check is unaffected

[safedep]

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