Background camera drag: inertial right-mouse mode

[malisimat]

When dragging on the background:

• Left mouse only should keep the current relative movement behavior.
• Right mouse down should switch to inertial mode, preserving current velocity and adding drag/resistance.
• Right mouse overrides left mouse while held.
• Releasing right while left remains held should return to relative mode.
• Motion should transition without jagged state swaps; momentum should carry across modes.
• Consider a max speed cap if needed.
• Right-only release should naturally decay to a stop.

Worktree/branch: feature/drag-smooth