feat(desktop): show the Modelflare logo on its provider preset

[NanmiCoder]

改动

桌面端「添加模型」的供应商列表里，Modelflare 赞助商 pill 之前只有名字和星标，没有 logo；AruHub 有。

• 新增 docs/images/sponsors/modelflare-logo.png（取自 modelflare.dev 官方 /logo.png，512×512）。
• ProviderSettings.tsx 把写死的 preset.id === 'aruhub' 改为 SPONSOR_LOGOS 表，logo 与赞助商星标都由它驱动。

验证

• desktop: vitest run src/pages/settings/ProviderSettings.test.tsx — 30 passed（新增断言：Modelflare/AruHub pill 带各自 logo，Atlas Cloud 不带）
• Mutation：从表里删掉 modelflare，新断言失败
• desktop: tsc --noEmit 通过

[github-actions]

PR quality triage

Changed areas: area:desktop, area:docs

CLI core policy: No CLI-core policy block detected.

Missing-test policy: No missing-test policy block detected.

Coverage baseline policy: No coverage-baseline policy block detected.

CLI core files:

• none

Coverage policy files:

• none

Required check plan:

• See PR Quality / scope-plan; it is the single source of truth for selected jobs.

Test coverage signals:

• No obvious missing-test signal from changed paths.

Risk notes:

• No special risk notes from changed paths.

Hard merge gates come from the deterministic GitHub Actions contract lanes above.