Move backup fixtures to a shared root-level test-fixtures/ directory

[gcsecsey]

Related issues

• Related to STU-1866
• Related to STU-1870

How AI was used in this PR

Claude Code planned and implemented the change under my direction, and I reviewed the result.

Proposed Changes

The minimal backup fixtures added in #4057 are currently under apps/studio/e2e/fixtures/, which ties them to the Playwright desktop suite. The upcoming CLI-based import tests (STU-1870) need the same archives, and reaching into apps/studio/e2e/** from apps/cli tests would couple them to a directory the project is migrating away from.

This PR:

• Moves apps/studio/e2e/fixtures/backups/ to a root-level test-fixtures/backups/
• mirrors how playwright.config.ts and vitest.config.ts already sit at the root
• No behavior change

Testing Instructions

1. Confirm the E2E Tests CI jobs are green (import-formats.test.ts runs there and covers all four backup formats)
2. Optionally verify no stale references remain: git grep -n "fixtures/backups" should only match test-fixtures/backups paths.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[wpmobilebot]

📊 Performance Test Results

Comparing 514e1f5 vs trunk

app-size

Metric	trunk	514e1f5	Diff	Change
App Size (Mac)	1419.29 MB	1419.29 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	514e1f5	Diff	Change
load	1070 ms	1087 ms	+17 ms	⚪ 0.0%

site-startup

Metric	trunk	514e1f5	Diff	Change
siteCreation	6996 ms	6996 ms	0 ms	⚪ 0.0%
siteStartup	2362 ms	2351 ms	11 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[gavande1]

Moving fixtures makes sense. LGTM 👍