Always merge both gateway and local (Claude Code, Codex, Hermes) sessions
in the sessions API instead of gating on `include_local`. The conversation
list now renders all sources unconditionally, using each session's `source`
field for display logic rather than the global `dashboardMode`.
- Add `localSessionsAvailable` store flag, set from capabilities `claudeHome`
- Remove `include_local` query param and early-return in sessions API
- Remove source-based filter and `dashboardMode` branching in conversation list
- Show both gateway and local active/recent groups when data exists