[feat] Eye-follow on hover (deferred polish from v0.1 spec)

[ikseong00]

Goal

When the cursor hovers over the pet, the pupils track the cursor position with a small offset.

Why

Deferred from the v0.1 spec (docs/specs/2026-05-15-pet-overlay-ui-design.md section "Hover"). Small detail but it makes the pet feel alive.

Acceptance

• When cursor is inside the pet window and over the pet sprite, pupils translate 1-2px toward cursor.
• Disable when not hovering (pupils return to neutral position).
• Works across all 4 phase sprites (Egg might not have eyes — that's OK, just no-op).

Hints

• Add id="left-eye-pupil" / id="right-eye-pupil" (or similar) to each phase[N].svg.
• In renderer/components/Pet.tsx, on pointermove, compute cursor offset relative to pet center and apply transform: translate(...) to the pupil elements via inline style or a separate React effect.
• Cap the offset (e.g. ±2px) so eyes don't fly off the face.
• Use requestAnimationFrame throttle if it feels jittery.

Out of scope

• Eyes blinking, eyes squinting on click, etc — those would be neat but separate.