Add desktop profile management UI

[wesbillman]

Summary

• add a relay-backed desktop profile surface with Tauri bridge commands, shared types, and React Query hooks
• add a profile management sheet plus sidebar entry point for editing display name, avatar URL, and about
• extend desktop e2e mocks and Playwright coverage for profile editing

Validation

• pnpm typecheck
• pnpm check
• cargo check --manifest-path desktop/src-tauri/Cargo.toml
• pnpm build
• cargo clippy --workspace --all-targets -- -D warnings
• ./scripts/run-tests.sh unit
• pnpm test:e2e:integration --grep "updates the relay-backed profile from the sidebar"