fix: restore mouse state after ExecProcess

[veeceey]

Summary

• Fixes a bug where mouse events (cell motion and all motion tracking) stop working after returning from a process launched via tea.ExecProcess (e.g., spawning vim from a TUI program)
• ReleaseTerminal already saved and restored bracketed paste, alt screen, and focus reporting state, but mouse mode was missing from this save/restore cycle
• Adds mouse state tracking to the renderer and wires it through ReleaseTerminal/RestoreTerminal

Fixes #1424

What changed

• renderer.go: Added mouseCellMotionActive() and mouseAllMotionActive() to the renderer interface
• standard_renderer.go: Added mouseCellMotion and mouseAllMotion boolean fields, updated enable/disable methods to track state, added getter methods
• nil_renderer.go: Implemented the new interface methods (always return false)
• tea.go: Added mouseCellMotionWasActive and mouseAllMotionWasActive fields to Program; ReleaseTerminal now saves mouse state; RestoreTerminal now re-enables mouse mode (including SGR extended mode) when it was previously active
• screen_test.go / nil_renderer_test.go: Added tests for mouse state tracking

Test plan

• All existing tests pass (go test ./...)
• go vet ./... passes cleanly
• New TestMouseStateTracking test verifies enable/disable state tracking on the standard renderer
• Nil renderer tests updated to cover new interface methods
• Manual testing: run a Bubble Tea program with mouse enabled, exec an external process (e.g., vim), exit it, and verify mouse events still work

🤖 Generated with Claude Code

[veeceey]

Friendly ping - any chance someone could take a look at this when they get a chance? Happy to make any changes if needed.