feat(desktop): adopt rounded Phosphor icons

[IchenDEV]

Summary

• replace the desktop Hugeicons adapter, dependency, and generator configuration with individually imported Phosphor icons
• remove built-in fallback emoji from renderer defaults, Core skills and market entries, and canonical Scene fixtures
• centralize accessible Git ahead/behind indicators and cover the shared icon contract with tests

Verification

• bun test — 816 passed
• bun run lint
• bunx tsc --noEmit
• bun run build:renderer
• cargo test -p codetwo-core
• bun script/verify/docs.ts
• bun script/verify/sdlc.ts
• bun script/verify/sdlc.ts --worktree
• rendered browser review at 1280px light/dark and 900px narrow

Residual risk

• packaged Windows rendering was not exercised; authored extension and provider brand icons remain intentionally outside the shared Phosphor family.