AI-First Starting UX: Build from scratch

[lmac-1]

Adds the "Build from scratch" path from the new-workflow landing screen (part of the #4848 AI-first starting UX epic).

Clicking the "Build from scratch" card now:

1. Immediately creates and saves a workflow via a new Workflows.create_webhook_workflow/2: a webhook trigger, one "Transform Data" job on @openfn/language-common@latest, and an always-enabled edge — the same shape as the old "Event-based template".
2. Navigates to the canvas with ?trigger=<id> in the URL, so the trigger inspector ("What triggers this workflow?") is open on load.
3. Because the workflow is already persisted, the canvas opens in the normal saved state: no pink unsaved toolbar, Save button visible.

The build_from_scratch LiveView event is authorized with the role-scoped ProjectUsers :create_workflow policy, so viewers get a permission error flash instead of a new workflow.

Workflows are named "Untitled workflow", "Untitled workflow 1", etc. via a new shared Workflows.unique_workflow_name/2 — extracted from the identical private copies in workflow_channel.ex and new_workflow_component.ex, and now used by both this flow and the channel's validate_workflow_name. (The legacy component keeps its copy since that path is being deleted in this epic.) This also fixes a bug where clicking the card a second time in the same project failed on the name unique constraint.

The card is also protected against double-clicks: useActionLock ensures one build_from_scratch event per click-burst, the card is disabled while creation is pending, and the LiveView handler has a creating_workflow? guard as a server-side backstop.

Closes #4895

Validation steps

1. In a project, open the new collaborative workflow screen — the landing screen appears.
2. Click Build from scratch.
3. You should land on the canvas of a saved workflow named "Untitled workflow" with a webhook trigger connected to a "Transform Data" job. The trigger inspector should be open, and the toolbar should be in the normal saved state.
4. Go back to the workflow list — the workflow was persisted.
5. Repeat steps 1–2 in the same project: the second workflow should be created as "Untitled workflow 1" instead of failing.
6. Delete one of them and build from scratch again: the freed name is reused (deletion renames the old row to <name>_del).
7. Double-click the card rapidly: only one workflow should be created, and the card should grey out while creation is pending.
8. Log in as a project viewer and click Build from scratch: you should see "You don't have permission to create workflows" and no workflow should be created.

Additional notes for the reviewer

1. create_webhook_workflow/2 goes through the existing save_workflow/2 pipeline, so snapshots, lock_version, and audit behave like any other workflow save.
2. Failure to save shows a "Failed to create workflow" flash and keeps the user on the landing screen.

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