refactor: split internal/ui model into focused modules

[JanSmrcka]

Summary

• refactor internal/ui/model.go into focused modules for dispatcher, modes, workflows, and rendering; keep Model.Update as dispatcher-only
• add characterization tests for commit/stage/refresh/tick/push-confirm behaviors to protect refactor with TDD-first guardrails
• document new module boundaries via file-level comments and reduce internal/ui/model.go from 1272 to 196 lines

Closes #31