✨ feat: add SayGM icon and provider mapping

[markdavison]

💻 变更类型 | Change Type

• ✨ feat

🔀 变更说明 | Description of Change

Adds the official SayGM brand mark and maps the saygm provider ID so ProviderIcon provider="saygm" resolves correctly.

• Adds web and React Native Mono, Color, Text, Combine, and Avatar variants
• Uses the official mark published on saygm.com
• Uses SayGM yellow (#FFCE3F) with the black mark for the color/avatar variants
• Registers SayGM in the web and React Native provider mappings
• Adds the generated docs catalogue entry

This is required by the SayGM LobeChat provider integration; without the mapping, LobeChat displays the generic fallback provider icon.

✅ Verification

• npm run lint
• npm run type-check
• npm run lint --workspace @lobehub/icons-rn
• npm run lint:circular
• npm run build
• Visual verification in LobeChat: provider avatar, combined heading, API-key form, and model list