Save and restore main Talk window position and size

[xhon-pelushi]

Summary

• Persist the main Talk window bounds in app config while moving/resizing/closing.
• Restore saved bounds on reopen when they are valid and still visible on a display.
• Fall back to the default size when saved bounds are missing or off-screen.

Fixes #15

Test plan

• Open Talk Desktop, move and resize the window, quit, and reopen — position/size should restore.
• Disconnect/move off a secondary display and reopen — should not open off-screen.
• Fresh config without saved bounds still opens at the default size.