fix: title bar disappears when all chats are cleared (#2729) (#689)

Navigate to home page after clearing session list if the current session
was deleted, and wrap the "session not found" fallback in a Page component
to ensure window controls remain visible on Linux/Windows.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
(cherry picked from commit 49c012b19e1d05e75a50f4f1c7c597a0a2d5ff57)