fix(projects): sync sidebar source-folder order with project settings

[im-ian]

Summary

Keep a multi-root project source-folder order consistent between the sidebar and Project Settings.

1. Persist sidebar source order — extract repository-root rows from the mixed sidebar DnD order and optimistically update Project.source_paths.
2. Add a guarded backend reorder command — reorder only roots already attached to the project, ignore unknown paths, and preserve omitted roots without changing project membership.
3. Cover the full flow — add store success/rollback tests, a Rust membership guard test, and a Playwright DnD-to-Project-Settings regression test.

Expected impact

Area	Before	After
Sidebar source-folder DnD	Changed only the sidebar-local mixed item order	Also persists the attached source-root order
Project Settings	Could retain the original source-folder order	Reflects the order chosen in the sidebar
Persistence failure	No backend persistence path existed	Optimistic state rolls back and surfaces the error

Design notes

• Project.repo_path remains the primary-root identity; only existing extra roots in source_paths are reordered.
• The sidebar filters its mixed session/workspace list down to default repository-root rows before calling the reorder action, so named workspaces and sessions cannot enter the source-root contract.
• The backend appends any omitted existing roots and ignores unknown or duplicate input paths, preventing reorder requests from changing project membership.

Test plan

• pnpm run typecheck
• pnpm exec vitest run src/store.test.ts src/lib/sidebarProjectItems.test.ts — 186 passed
• cargo test --manifest-path src-tauri/Cargo.toml reordering_project_sources_preserves_membership — 1 passed
• pnpm exec playwright test tests/e2e/project-settings.spec.ts — 7 passed
• pnpm run build
• rustfmt --edition 2021 --check src-tauri/src/commands.rs

Notes

• A production-profile local preview was built and launched successfully from commit a2494f8.