Add presentation-only camera.zoom so standing motions fit the canvas

The rig is ~211 canvas units tall standing but the canvas has 152 above
the ground line - every motion to date is seated or on the floor. A
per-motion "camera": {"zoom": ...} now scales the drawn output (figure,
props, mat, stroke widths) about the ground-center anchor in both the
reference renderer and the in-app view. Pure view transform: pins, prop
coordinates, and the Swift-solver fixtures stay in full-size authored
units; zoom 1 is byte-identical to before.

Claude-Session: https://claude.ai/code/session_01HJDQQDA9QdP8zByg43H5v3

Workouts/ExerciseFigure/ExerciseMotion.swift

@@ -106,6 +106,10 @@ struct MotionCamera: Codable {
     let yaw: Double?
     /// Camera elevation override; nil uses the standard slightly-raised viewpoint.
     let pitch: Double?
+    /// Presentation-only zoom about the ground-center anchor (nil = 1). Standing
+    /// motions author full-size anatomy — taller than the canvas — and zoom out
+    /// to fit; the solver and fixtures stay in full-size canvas units.
+    let zoom: Double?
 }
 
 /// A prop's joint reference: one joint (`"hand_r"`, `"knee_l"`, `"elbow_r"`, …)