added feature US-03: see trending agents

[snorouzinia]

Implemented a feature where the user can see trending agents.

Changes:

• Added a "Trending" nav link in frontend/src/App.tsx pointing to /trending.html.
• Added installs field to AgentSummary (backend/app/models/schemas.py).
• Implemented GET /agents/trending in backend/app/routers/agents.py reading agents/manifests and sorting by installs then rating.
• Placed /trending before the /{agent_id} route to avoid path collisions.