[FEATURE] Session list UX improvements: sorting, search, rename, and deletion

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

As a user managing many sessions, the current session list has several pain points:

1. No sorting by recent activity — hard to find the latest session
2. Search only matches titles, not conversation content
3. Rename reverts to auto-generated name after reload
4. Delete (trash icon) does not work reliably
5. No drag-to-reorder or pin sessions
6. Auto-generated titles use AI summarization instead of the user's first message, making sessions unrecognizable

Proposed Solution

• Add sort options (by last active time, by name)
• Make search index conversation content, not just titles
• Fix rename persistence across reloads
• Fix delete functionality
• Allow drag-to-reorder or pinning
• Use the user's first message as the default session title

Alternative Solutions

Currently using /rename manually, but it reverts after reload. No other workarounds available.

Priority

High - Significant impact on productivity

Feature Category

Other

Use Case Example

I manage 50+ sessions for different client projects. I need to quickly find a recent session, but the list has no sorting, search doesn't find content, and renamed sessions revert to unrecognizable auto-generated titles after reload.

Additional Context

_No response_