feat(dev-tools): add switch user option

[galierie]

This PR adds the "Switch to Another User" option in the Developer Tools.

[BastiDood]

Just a few more nitpicks and correctness issues.

[devin-ai-integration]

Devin Review found 1 new potential issue.

View 8 additional findings in Devin Review.

[devin-ai-integration]

🔴 Server uses error(404) instead of fail(404), so the client-side 'failure' toast is never triggered

When the target user email is not found, the server action calls error(404) at src/routes/dashboard/+page.server.js:316. In SvelteKit, error() throws an HttpError that produces result.type === 'error' in the use:enhance callback. However, the client-side form handler at src/lib/features/dev-tools/user-switcher/form.svelte:36 checks for result.type === 'failure' to display the toast.

Root Cause and Impact

SvelteKit distinguishes between two server-side error mechanisms:

• error(code) → result.type === 'error' → renders +error.svelte page
• fail(code) → result.type === 'failure' → stays on the page, accessible in the enhance callback

The form handler expects 'failure':

case 'failure':
  toast.error('Failed to switch to user.');
  break;

But the server throws error(404), which produces 'error', falling through to the default: break case. Combined with await update() on line 30, this navigates the user to the error page instead of showing a friendly toast. The codebase itself uses fail() elsewhere for recoverable business-logic errors (e.g., src/routes/dashboard/users/+page.server.js:93).

Impact: When a developer enters a non-existent email in the "Switch to Another User" dialog, they are unexpectedly taken to a 404 error page instead of seeing a toast notification.

Prompt for agents

In src/routes/dashboard/+page.server.js, two changes are needed:

1. On line 3, add `fail` to the import from '@sveltejs/kit':
   Change: import { error, redirect } from '@sveltejs/kit';
   To:     import { error, fail, redirect } from '@sveltejs/kit';

2. On line 316, change `error(404)` to `return fail(404)` so the client-side enhance callback receives `result.type === 'failure'` and can display the toast:
   Change: error(404);
   To:     return fail(404);

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[BastiDood]

Almost there! Sending in my preemptive approval.