feat(workflows): add JSON file download/upload for workflow transfer

From the PR description

Summary

Adds a way to move a workflow between Mike instances by exporting it as a .mikeworkflow.json file and importing it elsewhere - useful for sharing built/customised workflows across teams, environments, or organisations without needing a shared account.

• Download - every workflow row in /workflows gets a small download icon that serialises the workflow to a JSON envelope (format: "mike.workflow", version: 1) with title, type, practice, prompt_md, and columns_config.
• Upload - a new upload icon in the page header opens a file picker, validates the envelope, and creates a new workflow under the current user.
• Scope is intentionally minimal: no backend, schema, or Supabase config changes. Upload reuses the existing POST /workflows endpoint via createWorkflow. Email sharing is untouched.

Files

• New: frontend/src/app/lib/workflowFile.ts - envelope builder, download helper, strict parser with typed errors.
• New: frontend/src/app/components/workflows/UploadWorkflowButton.tsx - file-picker button that creates a workflow from a parsed envelope and shows inline errors.
• Modified: frontend/src/app/components/workflows/WorkflowList.tsx - mounts the upload button next to + New, adds a per-row download icon, widens the trailing actions column from w-8 to w-16.

No new dependencies. All icons come from the existing lucide-react import.

Test plan

• Create a custom assistant workflow with a prompt and practice → click the row's download icon → verify a <title>.mikeworkflow.json file downloads and the JSON contains format: "mike.workflow", version: 1, and the expected fields.
• Click the header upload icon → pick the downloaded file → verify a new workflow appears in the list with identical fields, owned by you.
• Repeat with a tabular workflow → verify columns_config round-trips.
• Upload an arbitrary JSON file → expect a red inline error from the envelope validator.
• Upload a malformed envelope (wrong format) → expect a clear inline error.
• Confirm no regression to existing flows: + New, email share modal, hide/unhide built-ins, bulk delete.

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