feat: sunsetting legacy editor

[doc-han]

Description

This PR sunsets the legacy editor

Closes #4402

Validation steps

Manual QA — Legacy editor sunset

Redirects

• /projects/:id/w/:wid/legacy redirects to the collaborative editor
• /projects/:id/w/new/legacy redirects to the new-workflow flow
• Legacy URL with query params still loads (no 404/error)

Legacy users

• A user previously on the legacy editor now lands on the collaborative editor
• No "switch to legacy editor" control anywhere
• No legacy deprecation warning shown

Collaborative editor (regression)

• Open existing workflow (diagram, nodes, edges render)
• Create workflow (template + from scratch), save works
• Add/edit/delete job, trigger, edge — persists after reload
• Real-time sync works across two browsers
• Job editor (Monaco) edits save
• Manual run: pick input, run, see result on canvas
• Credentials modal works
• Webhook auth modal works

AI Assistant

• Panel opens
• Job code mode returns a streamed answer
• Workflow template mode generates/edits a workflow
• Chat history loads + paginates
• Disclaimer prompt works for a new user
• Retry a failed message

Additional notes for the reviewer

1. (Is there anything else the reviewer should know or look out for?)

AI Usage

Please disclose whether you've used AI anywhere in this PR (it's cool, we just
want to know!):

• I have used Claude Code
• I have used another model
• I have not used AI

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Pre-submission checklist

• I have performed an AI review of my code (we recommend using /review
  with Claude Code)
• I have implemented and tested all related authorization policies.
  (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR

[github-actions]

Security Review ✅

• S0 (project scoping): LegacyRedirectController (lib/lightning_web/controllers/legacy_redirect_controller.ex:15-21) only issues a local redirect and reads no project data; the redirect target WorkflowLive.Collaborate still enforces :project_scope via on_mount.
• S1 (authorization): N/A — the only new endpoint is a redirect gated by [:browser, :require_authenticated_user] (lib/lightning_web/router.ex:146,261-262); authorization continues to be enforced at the redirect target, and the removed switch_to_legacy_editor handler is dead code with no policy surface.
• S2 (audit trail): N/A — the migration (priv/repo/migrations/20260629143825_clear_prefer_legacy_editor.exs) drops a per-user UI preference key, not a project/instance configuration resource that requires an audit entry.