fix: #3616336 Fix missing Donate h1 and Our Impact figure contrast

Fixes #3616336.

What/why: two accessibility slips on shipped Canvas pages.

• content/canvas_page/10-donate.yml: the page-title heading component was level: 2, so /donate had no h1 (axe page-has-heading-one). Now level: 1, matching the other 7 shipped pages.
• content/canvas_page/07-impact.yml: the three headline figures ("20K+", "137K+", "11K+") used number_color: text-primary, which resolves to the brand yellow #ffc72c on white — 1.56:1 measured, failing AA (axe [serious] color-contrast). Now number_color: text-black, matching the same figures on Home and Countries. Measures 21:1 after the fix.

Verified in a real browser: /donate renders h1 Donate; the Impact figures pass contrast. The Horizon Aid functional testing suite's accessibility scenarios (issue #3615987, still in progress) cover both defects and are what will gate this going forward.

AI-Generated: Yes

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