WEBDEV-9250: Size the topnav from its own scale unit

[jbuckner]

Converts the topnav's rem sizing (every size assumed a 10px html root, so it only rendered right on pages that set html { font-size: 10px } like archive.org and offshoot) to calc(N * var(--topnavUnit--)).

--topnavUnit-- is keyed off the browser's own default font size, not the page's root font-size: ia-topnav's :host sets font-size: var(--topnavFontSize, medium) (medium is the browser default, ignoring any html rem reset) and --topnavUnit--: calc(1em / 16), registered as a typed <length> custom property so it resolves once and inherits as a fixed px value through nested shadow roots instead of compounding. A JS fallback covers browsers without CSS.registerProperty (old Firefox/Safari).

Result: the topnav renders pixel-identical to today at the browser's 16px default, with no consumer override needed anywhere, and it grows when the user raises their default font size (confirmed in Chromium, Firefox and WebKit). Neither petabox nor offshoot needs a change for this.

Jira: https://webarchive.jira.com/browse/WEBDEV-9250

🤖 Generated with Claude Code

[github-actions]

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🚀 View preview at https://internetarchive.github.io/elements/pr/pr-137/
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[codecov-commenter]

Codecov Report

❌ Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.54%. Comparing base (61aa77a) to head (15f26a6).

Files with missing lines	Patch %	Lines
src/elements/ia-topnav/lib/register-topnav-unit.ts	54.54%	3 Missing and 2 partials ⚠️
src/elements/ia-topnav/ia-topnav.ts	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                          Coverage Diff                          @@
##           WEBDEV-9221-localize-topnav-menus     #137      +/-   ##
=====================================================================
- Coverage                              89.69%   89.54%   -0.16%     
=====================================================================
  Files                                     83       84       +1     
  Lines                                   3009     3022      +13     
  Branches                                 717      722       +5     
=====================================================================
+ Hits                                    2699     2706       +7     
- Misses                                   135      138       +3     
- Partials                                 175      178       +3     

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