fix(theme): make navbar width consistent with main layout on large di…

[octacava]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes Making navbar width consistent with the main layout on large displays #7562) and the maintainers have approved on my working plan.

Motivation

Fixes #7562
On large displays, the navbar width expands to the edges of the screen, making it inconsistent with the main content area and footer which have a maximum width. This PR limits the navbar__inner maximum width to match the rest of the layout (using var(--ifm-container-width-xl)) and centers it, improving readability and scanning on ultrawide monitors.
(AI-assisted)

Test Plan

Tested locally by running the Docusaurus website and expanding the browser window to an ultrawide resolution. Verified that the navbar contents remain centered and aligned with the main page content layout.

Test links

N/A

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Related issues/PRs

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