fix(render): use an isolated temp workspace instead of deleting ./frames

[ubugeeei]

Finding

The renderer uses a cwd-relative frames directory and deletes/recreates it. This risks deleting user data, colliding across parallel renders, and breaking packaged app assumptions.

Evidence

• render/src/main.rs creates/removes ./frames.
• .gitignore also ignores frames, implying local generated output.

Acceptance criteria

• Use a per-render temp directory.
• Clean it up on success/failure/cancel.
• Avoid cwd-dependent destructive deletion.

[ubugeeei]

対応済みです。修正は PR #1 / commit 9cf4498 に反映済みです。