refactor(themes): convert all rgba() to #RRGGBBAA hex notation

453 occurrences, 37 unique rgba() values converted across 66 files.
All 110 templates (10 themes × 11 types) render successfully.

Co-Authored-By: Claude <noreply@anthropic.com>

themes/terra/mail/repo/collaborator.tmpl

@@ -16,7 +16,7 @@
 
     /* Header */
     .email-logo { display:block;margin:0 auto 10px;border:0;outline:none }
-    .email-brand { font-size:20px;font-weight:700;color:#fff;letter-spacing:1px;text-shadow:0 1px 2px rgba(0,0,0,0.1) }
+    .email-brand { font-size:20px;font-weight:700;color:#fff;letter-spacing:1px;text-shadow:0 1px 2px #0000001a }
     .email-header { padding:0;background:linear-gradient(180deg,#c67b5c,#d4956e,#e8c4a8);height:100px;text-align:center;vertical-align:bottom }
 
     /* Card */