fix(Modal): use fixed positioning to avoid parent-dependent sizing

[huyenltnguyen]

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#744 works but it causes Playwright failure:

62) [chromium] › video-modal.spec.ts:14:7 › Exit Video Modal E2E Test Suite › Verifies the Correct Rendering of the Video Modal 

    Error: Timed out 5000ms waiting for expect(locator).toBeVisible()

    Locator: getByRole('dialog', { name: 'Watch a Video' })
    Expected: visible
    Received: hidden
    Call log:
      - expect.toBeVisible with timeout 5000ms
      - waiting for getByRole('dialog', { name: 'Watch a Video' })
        9 × locator resolved to <div role="dialog" aria-modal="true" id="headlessui-dialog-6" data-headlessui-state="open" class="relative z-1050 w-full h-full" aria-labelledby="headlessui-dialog-title-8">…</div>
          - unexpected value "hidden"


      17 |     await expect(
      18 |       page.getByRole('dialog', { name: translations.buttons['watch-video'] })
    > 19 |     ).toBeVisible();
         |       ^
      20 |     await expect(
      21 |       page.getByTestId('video-modal-video-player-iframe')
      22 |     ).toBeVisible();
        at /home/runner/work/freeCodeCamp/freeCodeCamp/e2e/video-modal.spec.ts:19:7

https://github.com/freeCodeCamp/freeCodeCamp/actions/runs/21824605372/job/62967345618?pr=65786

This is because the internal Dialog uses relative w-full h-full, which can collapse when the portal parent has no explicit size.

---

The fix is to change to fixed inset-0:

• position: fixed removes the Dialog from normal document flow and anchors it to the viewport (not its parent).
• inset: 0 sets top/right/bottom/left: 0, so the Dialog exactly covers the viewport regardless of parent sizing or page scroll state.
• Because it doesn't participate in the page's layout, it does not add to the scrollable area and doesn't introduce extra scrollbars or shifts caused by scrollbar width.

Screen recording (modal opens and closes correctly, without injecting scrollbars)

Screen.Recording.2026-02-10.at.01.15.30.mov