PER-10647: Stripe sketch for design

[cecilia-donnelly]

This walks through the new PaymentIntent Stripe flow, with stela endpoints instead of donations-firebase. It's mainly for the purpose of getting design feedback. Needs review and updates since it was written by Claude.

[codecov]

Codecov Report

❌ Patch coverage is 80.19802% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.31%. Comparing base (9a5abe9) to head (f34e1f9).
⚠️ Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
...yment-intent-form/payment-intent-form.component.ts	74.19%	10 Missing and 6 partials ⚠️
...intent-confirm/payment-intent-confirm.component.ts	85.71%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1114      +/-   ##
==========================================
+ Coverage   52.17%   52.31%   +0.14%     
==========================================
  Files         354      356       +2     
  Lines       12090    12189      +99     
  Branches     2186     2198      +12     
==========================================
+ Hits         6308     6377      +69     
- Misses       5557     5580      +23     
- Partials      225      232       +7     

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