fix(projects): refresh replaced avatar images

[fsioni]

Fixes

Replacing an avatar image didn't work, even after removing the old one

Summary

• store each imported project avatar under a fresh UUID-based filename
• remove legacy and previous avatar files before copying the replacement
• add frontend and Rust regression coverage for avatar replacement caching

Testing

• bun --bun node_modules/vitest/vitest.mjs run src/components/projects/panes/GeneralPane.test.tsx
• cargo test --manifest-path jean-core/Cargo.toml project_avatar_replacements_get_a_fresh_file_name
• bun run typecheck
• bun run lint

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