Media composition, safer deletion and configurable studio identity

[GodsQuantum]

Summary

• add aspect-safe image/GIF fit, anchors and ratio-aware resizing
• add procedural shooting-star overlay with renderer/scheduler support
• add static splash composition endpoint and UI controls
• add media usage reporting plus detach-and-delete flow
• make the admin studio name use the existing display brand setting

Verification

• frontend: 19 files / 91 tests
• svelte-check: 0 errors / 0 warnings
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --check
• cargo audit / cargo deny check
• deployment + repo hygiene tests
• full Podman OCI build + OCI smoke + runtime executable scan
• independent Codex review: no actionable correctness issues