feat(web-ui): make installed capabilities more informative

[Minitour]

Summary

• Fall back to SKILL.md frontmatter when a skill has no capabilities-file description, and expose skill content via a new API endpoint
• Add a skill detail dialog with rendered markdown, file tree, and View source links for github/gitlab/remote/plugin skills
• Group configured tools by MCP server / command group, and constrain capability lists to a scrollable max height

Test plan

• Open a project detail page with installed skills/tools/servers
• Confirm skills without def.description show frontmatter descriptions
• Click a skill and verify markdown, file tree, and View source (when applicable)
• Confirm tools are grouped by server/group
• Confirm long capability lists scroll within ~520px height