feat: persist editor mode selection between plan reviews

[rcdailey]

Summary

• Adds persistent state for the editor mode (Selection, Comment, Redline) buttons
• The last selected mode is now remembered between plan reviews using cookie-based storage
• Follows the existing pattern used for taterMode, agentSwitch, and other settings

Changes

• New packages/ui/utils/editorMode.ts utility with getEditorMode() and saveEditorMode() functions
• Updated packages/editor/App.tsx to initialize state from storage and save on mode change

[backnotprop]

I just got back from my only vacation this year and will get these PRs going.

[rcdailey]

Shame on you for enjoying a vacation instead of slaving away on PRs!

I'm kidding 🤣

Thank you and welcome back!