Fix speed clamp in diff-drive backend

Description

Fix the speed normalization logic so robot drive speed is correctly clamped from 0-100 and mapped to 0.0-1.0

Acceptance criteria

• Speed clamp formula is corrected in server/main.py.
• Inputs below 0 become 0, above 100 become 100.
• Normalized speed sent to motion command is between 0.0 and 1.0.
• Basic tests cover boundary and invalid values.

Developer note

• Current logic always returns 100