Fix nav bar overlapping the top of rendered notebook pages

[e-koch]

Summary

• Follow-up to Add site header to rendered notebooks and issue-tracker link #51/Add gap below nav bar on rendered notebook pages #53: on the deployed notebook pages the injected nav bar still covers the first cell (the "Questions or issues?" note). The body { padding-top: 60px } rule from Add gap below nav bar on rendered notebook pages #53 never took effect.
• Root cause: nbconvert's lab template renders <body class="jp-Notebook">, and its stylesheet includes .jp-Notebook { padding: var(--jp-notebook-padding); } (10px). A class selector outranks a bare body selector regardless of source order, so the computed padding-top was 10px, and the 42px absolutely positioned nav overlapped the content.
• Fix: match body.jp-Notebook as well (body, body.jp-Notebook { padding-top: 60px; }), with a comment explaining why the class is needed.

Test plan

• Rendered fitting_with_spectralcube.ipynb locally (nbconvert 7.16.4, not executed), ran insert_header.inject() on it, and measured the layout in headless Chromium (Brave) at 1400px and 600px widths:

  	computed body padding-top	nav bottom	first blockquote top
  before	10px	42px	29px (13px under the nav)
  after	60px	42px	79px (37px clear)

• After merge, confirm on the live site that the "Questions or issues?" note is fully visible below the nav.

Note: CI's nbconvert version isn't pinned (it comes in via notebook), so a future template that renames the <body> class could bring this back.

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