feat(ui): enhance empty states for skills and task run history

[xiaoye5200]

Summary

Add icon and subtitle to minimal empty state placeholders in SkillsManager (installed/marketplace tabs) and TaskRunHistory, matching the richer style used in CoworkSessionList.

Related Issue

Closes #1921

Changes Made

• Add PuzzleIcon + subtitle to SkillsManager installed tab empty state
• Add PuzzleIcon + subtitle to SkillsManager marketplace tab empty state
• Add ClockIcon + subtitle to TaskRunHistory empty state
• Add i18n hint keys for all new subtitle text (zh + en)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation update
• Performance improvement
• Other (please describe):

Testing

• Tested locally
• Added new tests
• Updated existing tests
• Manual testing performed

Screenshots (if applicable)

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Electron-Specific Changes

• Changes to main process (src/main/)
• Changes to preload script (src/main/preload.ts)
• Changes to IPC communication
• Changes to window management
• None

Additional Notes

Pure UI change, zero business logic touched. Uses existing PuzzleIcon and ClockIcon already in the project.

[xiaoye5200]

Hi @btc69m979y-dotcom @fisherdaddy @liugang519 @liuzhq1986 👋

This PR adds icons and descriptive subtitles to the empty state placeholders in SkillsManager (installed & marketplace tabs) and TaskRunHistory, bringing them in line with the richer empty state style already used in CoworkSessionList. Pure UI change — no business logic touched.

A tracking issue has been created at #1921. Would appreciate a review when you get a chance — happy to address any feedback!

Thanks 🙏