fix(frontend): add nested error boundaries for app routes

[fpindej]

Problem

Only the root `+error.svelte` exists. When an error occurs in any app or admin page (e.g. a failed server load on the users list), the entire viewport - including sidebar, breadcrumbs, and header - is replaced with the root error page. This is disorienting and prevents navigation to other pages.

Split from #130, which also covers skip-to-content and focus management (independently deliverable).

Proposed Solution

Add `+error.svelte` files to key route groups:

• `(app)/+error.svelte` - keeps sidebar/header visible, shows error in the content area
• `(app)/admin/+error.svelte` - admin-specific error handling within the app shell

Both should reuse the existing error page patterns (status code display, descriptive messages, navigation back).

Affected Files

• New: `src/frontend/src/routes/(app)/+error.svelte`
• New: `src/frontend/src/routes/(app)/admin/+error.svelte`
• Reference: `src/frontend/src/routes/+error.svelte`