WEBDEV-9052: Migrate edit-donation into elements

[jbuckner]

WEBDEV-9052. Stacked on #98. Third of the five in WEBDEV-8602.

donation-form-edit-donation moves over as ia-donation-edit-donation, with its story and tests on Vitest. The petabox banner uses this one standalone, so it has its own catalog entry.

What changed beyond the move:

• It renders ia-donation-section for its two steps instead of donation-form-section, and the enums are as const objects plus a type.
• CSS variables move to --ia-donation-edit-* (button colors, border, gap, font sizes, custom amount width, input border, badge and background pass-throughs to the sections). Everything is sized off a 10px base font size variable, which is also handed to the sections so the whole thing scales together. The two layout variables the element sets on its own host from the number of amounts are renamed too (--ia-donation-edit-amount-column-count, --ia-donation-edit-custom-amount-col-span). The old --editFormTextColor pass-through is gone, the section reads --ia-theme-primary-text-color directly.
• Strings go through msg(), and the custom amount text field has an aria-label, it only had the visible "Custom: $" text beside it before.
• currency.js is a new dependency, for the amount formatting.
• One inherited bug fixed. With customAmountMode="hide", the "is the donor typing in the custom field" check compared null to null and came out true, so a preset set programmatically (say the banner switching amounts) could leave nothing selected until the donor clicked. Guarded on the field existing, with a test that fails on the old code.

Verified locally: build, full suite 706 passing, eslint and prettier clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01QnX6VHo3EJRzcKfhzBChCf

[github-actions]

PR Preview Action v1.8.1
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[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.83%. Comparing base (799d264) to head (8744ccc).

Files with missing lines	Patch %	Lines
...on-form/form-elements/ia-donation-edit-donation.ts	92.09%	6 Missing and 8 partials ⚠️
...m/form-elements/ia-donation-edit-donation-story.ts	61.53%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@                             Coverage Diff                              @@
##           WEBDEV-9051-migrate-donation-form-models     #100      +/-   ##
============================================================================
+ Coverage                                     86.58%   86.83%   +0.25%     
============================================================================
  Files                                            76       78       +2     
  Lines                                          2340     2530     +190     
  Branches                                        501      548      +47     
============================================================================
+ Hits                                           2026     2197     +171     
- Misses                                          168      178      +10     
- Partials                                        146      155       +9     

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