Apply identity default Bcc in compose

[keilogic]

Fixes #442

Summary

• adds the identity default_bcc field to the profile editor create/update flow
• applies an identity's default Bcc recipients to new drafts, replies, and forwards
• keeps compose Bcc recipients synchronized when the From identity changes, and refreshes them before send validation

Tests

• Pre-fix check: npx ng test --include src/app/compose/draftdesk.service.spec.ts --watch=false --browsers=FirefoxHeadless failed as expected with the new default-Bcc expectations (TOTAL: 3 FAILED, 22 SUCCESS).
• npx ng test --include src/app/compose/draftdesk.service.spec.ts --watch=false --browsers=FirefoxHeadless (TOTAL: 25 SUCCESS)
• npx tsc -p src/tsconfig.app.json --noEmit
• npx tsc -p src/tsconfig.spec.json --noEmit
• npm run lint -- --quiet
• npm run policy (passes for these commits; still reports existing historical upstream commit-message warnings)
• git diff --check origin/master...HEAD
• SKIP_CHANGELOG=1 production build sequence: node src/build/pre-build.js, npx ng build --configuration production --base-href=/app/ runbox7, node src/build/post-build.js (build hash 68316653f4a23fc8; existing Angular/CommonJS/theming warnings)

The regression tests are in a separate commit from the implementation commit.

AI disclosure

I used OpenAI Codex as an AI coding assistant to inspect the relevant compose/profile code, implement the patch, and run the validation commands listed above.