feat(editor): add cursor opacity and circle color controls

[soaapp]

Summary

• Adds an Opacity slider (0–200%, default 100%) to the Studio Mode editor's Cursor tab. Multiplied into the existing cursor_opacity shader uniform so it composes correctly with the existing idle-fade behaviour and applies to every cursor type.
• Adds a Circle Color picker (default #FFFFFF) that tints the baked Circle cursor texture. Only rendered when Cursor Type is Circle.
• Two new fields on CursorConfiguration (cursor_opacity: f32, circle_color: String) with #[serde(default = "...")] attributes so existing project files load unchanged.

Why

Today the Circle cursor is a fixed low-opacity grayscale halo (hard-coded fill_alpha = 0.2, border_alpha = 0.55 in crates/rendering/src/layers/cursor.rs), which can be too translucent to see against many recordings. These two controls let users push visibility up (more opacity, a saturated tint) without touching defaults.

Implementation notes

• Renderer (crates/rendering/src/layers/cursor.rs):
  • create_circle_cursor now takes a [f32; 3] tint and bakes it into the premultiplied RGB of the texture.
  • Added prev_circle_color to CursorLayer and an invalidation check next to the existing prev_cursor_type check, so the texture is re-baked exactly when the color changes.
  • User opacity is multiplied into the existing cursor_opacity path (which already drives the motion_vector_strength.w shader uniform), so the WGSL shader and pipeline layout are unchanged.
• UI (apps/desktop/src/routes/editor/ConfigSidebar.tsx): two new Fields placed under Size in the Cursor tab; the color picker is wrapped in <Show when={project.cursor.type === "circle"}>.

Test plan

• Open a recording in Studio Mode → Cursor tab. New Opacity slider visible under Size.
• Drag opacity to 0% → cursor disappears in the preview; back to 100% restores it.
• Drag opacity to 200% → cursor visibly more opaque (most obvious on the Circle type).
• Switch Cursor Type to Circle → Circle Color picker appears under Opacity.
• Pick a saturated color (red, yellow, cyan) → preview re-tints within a frame or two.
• Switch Cursor Type back to Auto / Pointer → Circle Color picker is hidden; Opacity still applies.
• Open a recording made before this change → loads with defaults (#FFFFFF, 1.0) applied via serde.

Screenshots

To be added.