Fixes a mobile WebUI regression in the provider configuration page.

When the provider source list collapses into a v-select on mobile screens, the delete action for provider sources disappears, which makes provider sources impossible to remove from mobile layout. This PR restores the delete entrypoint for the currently selected provider source without changing existing backend logic or desktop behavior.

修复 WebUI 提供商配置页面在移动端下的一个回归问题。

当提供商源列表在移动端折叠为 v-select 下拉框后，删除提供商源的入口丢失，导致用户无法在移动端删除提供商源。本 PR 恢复了当前选中提供商源的删除入口，同时不修改现有后端逻辑，也不影响桌面端行为。

Modifications / 改动点

• Updated dashboard/src/components/provider/ProviderSourcesPanel.vue

• Added a mobile-only delete button next to the provider source v-select

• Only shows the delete button when a real provider source is selected (not a placeholder item)

• Reused the existing delete-provider-source event flow and deletion logic in useProviderSources.ts

• Kept desktop deletion behavior unchanged

• 修改了 dashboard/src/components/provider/ProviderSourcesPanel.vue

• 在移动端提供商源 v-select 旁边补回删除按钮

• 仅在当前选中真实提供商源时显示删除按钮（不会对 placeholder 显示）

• 复用了现有 delete-provider-source 事件链路和 useProviderSources.ts 中的删除逻辑

• 保持桌面端删除行为不变

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Restore mobile WebUI ability to delete provider sources when the source list is rendered as a select on small screens.

Bug Fixes:

• Reintroduce a delete action for the currently selected provider source in the mobile layout without affecting desktop behavior.

Enhancements:

• Add conditional logic and styling to only display the mobile delete button for non-placeholder provider sources.