[design] Replace placeholder SVG sprites with proper art

[ikseong00]

Goal

Replace the hand-coded SVG sprites at renderer/public/sprites/phase[0-3].svg with real character art.

Why

Current sprites are visibly placeholder ("대충 만든 상태" per the docs). The whole "cute desktop pet" pitch lives or dies on this. First impression problem.

Acceptance

• 4 SVGs: phase0.svg (Egg), phase1.svg (Baby), phase2.svg (Middle), phase3.svg (Final).
• viewBox 128x128, transparent background.
• Visible at 120px window size (test min size) — outline thick enough.
• mood filter (CSS brightness/saturate) doesn't kill readability when applied (test sleepy/sad states).
• (Optional / bonus) per-mood overlay sprites, since today we only use emoji 💤😔 — but emoji-only is acceptable for v1.

Hints

• docs/05-pet-design.md has an "art replacement guide" with full checklist.
• Same filenames → no code change needed.
• drop-shadow CSS already applied (.pet { filter: drop-shadow(...) }) — leave room around sprite edges.
• Style direction: TBD by designer. Loose constraint: small/cute/kawaii.

Out of scope

• Animated sprites (mp4/lottie). Stay SVG static; CSS keyframes handle animation.