feat(compose): move recipients between fields

[isaacsamual994-lang]

Summary

• make existing To/CC/BCC recipient chips draggable
• preserve the source field during drag so dropping moves the recipient instead of copying it
• avoid duplicate recipients when dropping onto the same or target field

Closes #132.

Testing

• Added regression tests before the implementation; both failed before the fix and pass afterward.
• Targeted recipient drag/drop tests: 2 passed.
• Full unit suite: 259 passed (2 skipped).
• ng lint runbox7: 0 errors (existing repository warnings remain).
• Production build: passed.

I also ran the repository CI wrapper locally. Its Cypress phase could not start Cypress 12.17.4 on the local darwin-arm64 environment (Cypress.app: bad option: --no-sandbox); the unit phase completed successfully before that environment-specific launcher failure.

[isaacsamual994-lang]

Implementation is ready for review. I re-verified the current PR branch locally: the targeted recipient drag/drop regression tests pass (2/2), ng lint runbox7 completes with 0 errors (existing repository warnings only), and the production build passes. GitHub currently shows both workflows as awaiting maintainer approval and the PR as requiring one approving review. Could a maintainer please approve the workflows and review the PR when convenient? Also, could you confirm whether #132 is eligible for the Runbox 7 Feature Bounty Program and which reward tier applies?