feat(mail): add send again action

[keilogic]

Fixes #404.

Summary

• Add a Sent-folder-only Send again action to the opened-message toolbar and overflow menu.
• Create a new unsaved compose draft from the sent message with the original From identity when available, To/Cc/Bcc recipients, subject, plain/HTML body, and copied attachments.
• Suppress automatic signature insertion for resend drafts so the sent message content is not modified before the user edits it.
• Parse Bcc from opened message contents and add focused regression coverage for resend draft creation and Sent-folder message parsing.

Validation

• npm ci (passes; existing dependency deprecation/audit warnings)
• npx tsc -p src/tsconfig.app.json --noEmit
• npx tsc -p src/tsconfig.spec.json --noEmit
• npx ng test --watch=false --browsers=FirefoxHeadless --include src/app/compose/draftdesk.service.spec.ts (24 SUCCESS)
• npx ng test --watch=false --browsers=FirefoxHeadless --include src/app/mailviewer/singlemailviewer.component.spec.ts (8 SUCCESS; existing mocked attachment thumbnail 404)
• npx eslint src/app/compose/draftdesk.service.ts src/app/compose/draftdesk.service.spec.ts src/app/compose/compose.component.ts src/app/mailviewer/messageactions.ts src/app/mailviewer/rmm6messageactions.ts src/app/mailviewer/rmm7messageactions.ts src/app/mailviewer/singlemailviewer.component.ts src/app/mailviewer/singlemailviewer.component.spec.ts src/app/mailviewer/singlemailviewer.component.html --quiet
• npm run lint -- --quiet
• git diff --check
• git diff --cached --check
• npx ng test --watch=false --browsers=FirefoxHeadless (247 SUCCESS, 2 skipped; existing console output from older specs)
• npm run policy (passes for the current commit; historical upstream commit-message warnings are still printed)
• $env:SKIP_CHANGELOG='1'; node src/build/pre-build.js; npx ng build --configuration production --base-href=/app/ runbox7; $buildExit=$LASTEXITCODE; node src/build/post-build.js; if ($buildExit -ne 0) { exit $buildExit } (passes; existing Angular/CommonJS warnings; production build hash 3a314214b1facc2f)

AI disclosure

I used OpenAI Codex (GPT-5) in a local development session to inspect the Runbox 7 codebase, implement this patch, and run validation. I reviewed the generated changes and test/build outputs before submitting this PR.