feat: onboarding personality + name step with dynamic email domain

[AggManik]

Summary

• Two-step onboarding at /onboarding/name: personality selection (5 presets + custom "build your own") then name/email form
• Auto-generates butler email as {butlerName}_{userName}@{domain} when a default personality is selected
• Removes hardcoded getcore.me; derives email domain from LOGIN_ORIGIN env var at runtime
• Custom personality option prefills prompt with Alfred voice guide; saves to workspace metadata on submit
• Saves personality selection to user.metadata.personality on completion
• Exports ALFRED_VOICE from personality.ts for reuse
• Adds utils/onboarding-email.ts with deriveEmailDomain, generateButlerEmailSlug, generateButlerEmail helpers and vitest unit tests

Test plan

• Select each of the 5 default personalities; verify step 2 pre-fills Name and auto-generates email
• Change Name field; verify email updates automatically
• Edit Email field directly; verify auto-generation stops
• Click "change personality" ghost button; verify returns to step 1 with state preserved
• Select custom (+); verify Name/Email empty, prompt pre-filled with Alfred content
• Submit custom; verify custom personality saved to workspace metadata and user.metadata.personality set to custom ID
• Confirm LOGIN_ORIGIN=https://app.example.com yields domain app.example.com in email
• Run pnpm --filter webapp test to confirm unit tests pass

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