Add configurable settings for freeview numpad camera

[Mottserrat]

Motivation

Adds configurable console variables for the numpad freeview camera movement (previously added in #5), so the movement can be enabled/disabled and its speed/smoothness tuned without recompiling:

• bc_freeview_numpad (enable/disable)
• bc_freeview_speed (movement speed, 1-1000)
• bc_freeview_smoothness (acceleration/smoothing)

Checklist

• Tested the change ingame
• Provided screenshots if it is a visual change
• Tested in combination with possibly related configuration options
• Written a unit test (especially base/) or added coverage to integration test
• Considered possible null pointers and out of bounds array indexing
• Changed no physics that affect existing maps
• Tested the change with ASan+UBSan or valgrind's memcheck (optional)
• I didn't use generative AI to generate more than single-line completions

AI was used to help write and debug the config variable integration and troubleshoot the build environment (Rust/dependency setup); the original camera movement logic itself was written by me in an earlier PR (#5).