test: reorganize tests into tests/ dir and add CI workflow

[ohxyz]

What

• Relocate tests — move all `*.test.js` from `src/` into a top-level `tests/` directory mirroring the `src/` layout. Sibling-relative imports rewritten to the `@/` alias (already configured in `vitest.config.js`).
• Fix manager tests — the refactor: event-emitter for pane lifecycle + detached/windowless glass overhaul #102 refactor turned `detached-glass/manager.js` into a pure registry (open/close animation, `opening`/`closing` attrs, and Promise returns moved to the crud layer). Updated the stale tests to assert the registry-only contract.
• Add CI — new `unit-tests` workflow runs the suite on PR open/synchronize/reopen.

Notes

• All 74 tests pass.
• `src/` source is unchanged; only test files moved + assertions updated.
• The `unit-tests` job can be added as a required status check once it runs here.