feat(dev): use toast error messages for failed DevTool actions

[BastiDood]

This PR adds granular error handling to all developer tool form actions (closes #143). Previously, DevTool failures threw opaque error() responses; now they use fail() with specific HTTP status codes (404, 422), enabling the client-side Svelte forms to display context-aware toast messages per failure type.

Also fixes a cyclical type reference in app.d.ts by importing from specific schema submodules instead of the barrel export.

Implementation Notes

• fail() over error(): Dev tool actions now return fail(status) or fail(status, message) instead of throwing error(status). This lets use:enhance callbacks discriminate on result.status and show targeted toasts.
• Database error discrimination: The role action catches DrizzleQueryError wrapping DatabaseError with code 23503 (foreign key violation) to surface a "Lab not found" toast when an invalid lab ID is provided.
• Valibot validation errors: All dev tool actions now catch ValiError and return fail(422) for malformed input, surfaced as "Invalid input" toasts on the client.
• Email action 404s: Each email event branch individually catches AssertionError from getUserNameByEmail/getLabById and returns fail(404, descriptiveMessage), allowing the client to show the specific missing entity in the toast.
• Dummy user creation: Wrapped in a REPEATABLE READ transaction for atomicity of user upsert + session creation/swap.

Test Cases

• Role switcher: switching to a non-existent lab shows "Lab not found" toast
• Role switcher: submitting invalid form data shows "Invalid role input" toast
• Dummy user creator: submitting invalid form data shows "Invalid dummy user input" toast
• User switcher: switching to a non-existent email shows "User email not found" toast
• User switcher: submitting invalid form data shows "Invalid user email input" toast
• Email dispatcher: referencing a non-existent lab/user shows specific 404 toast
• Email dispatcher: submitting invalid payload shows "Invalid email event payload" toast
• All dev tools: successful actions still work as before

[devin-ai-integration]

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