AI-First Starting UX: Landing screen shell for new workflow creation

[lmac-1]

Part of #4848 epic. Merges into 4848-ai-first-starting-ux-parent parent branch

What this adds

A React overlay that replaces the bare /w/new canvas. When a new user creates a new workflow, they see three options: Build with AI, Browse Templates and Import from YAML. Each downstream path is stubbed - callback props are wired, nothing is connected yet.

Closes #4856

Key files

• LandingScreen.tsx — the overlay component
• createUIStore.ts — showLandingScreen state + dismissLandingScreen command
• useUI.ts — useShowLandingScreen hook
• CollaborativeEditor.tsx — renders LandingScreenWrapper, gates the header

Design decisions

1. Header gated on !isNewWorkflow, not !showLandingScreen. The header must stay hidden while the user is on the "Build with AI" path (pink toolbar state) — showLandingScreen is already false at that point but no workflow exists yet. Gating on showLandingScreen would expose the header prematurely.
2. Landing screen is a React overlay, not a LiveView component. All downstream paths interact via React store calls (openAIAssistantPanel, etc.) and aiAssistantEnabled is already a prop on CollaborativeEditor. Staying in the same layer avoids a React/LiveView boundary at every handoff.
3. LandingScreenWrapper inner-component pattern CollaborativeEditor sits outside StoreProvider and can't call hooks directly. Same pattern as BreadcrumbContent — a small inner component rendered inside the provider reads the store and returns null or <LandingScreen>.
4. Enter submits, Shift+Enter / Alt+Enter inserts a newline — confirmed by product; textarea not single-line input.
5. Empty canvas placeholder suppressed while landing screen is visible. The overlay has a transparent background so the dot pattern shows through. Without suppression, the "Create your workflow" placeholder renders visibly behind it.
6. Modals open on top of the landing screen — Browse Templates and Import YAML overlays leave the landing screen visible underneath. The landing screen is only dismissed when a path is committed, not when a card is clicked.

Out of scope (intentional)

• Focus gradient on the AI textarea — deferred pending Figma dev mode access for exact CSS
• createWorkflowPanelCollapsed cleanup — scoped to AI-First Starting UX: Cleanup — remove old canvas entry points #4876
• Organising components in a generic way for new design system - to be revisited if there is time later

Additional notes for the reviewer

1. Not planning to have this PR reviewed as it's the first piece of work in a series of issues for this epic. Things may change.

AI Usage

Please disclose whether you've used AI anywhere in this PR (it's cool, we just
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• 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. To be done in final issue
• I have ticked a box in "AI usage" in this PR

[github-actions]

Security Review ✅

• S0 (project scoping): N/A — all changes are frontend React/TypeScript in assets/js/collaborative-editor/; no new queries, channel handlers, or endpoints touch project-scoped data.
• S1 (authorization): N/A — the new LandingScreen component (assets/js/collaborative-editor/components/LandingScreen.tsx) is a pure UI overlay with stub callbacks (onBuildWithAI/onBrowseTemplates/onImportYAML are () => {}), introducing no server-side actions.
• S2 (audit trail): N/A — no Repo.insert/update/delete or Ecto.Multi changes; createUIStore.ts only adds a client-side showLandingScreen flag.