refactor(confirm-service): normalise import spelling to no-extension form

Production code referenced $lib/shared/services/confirm.svelte under two
spellings — 4 files with the .js extension and one without. Standardise on
the no-extension form to match Svelte 5 rune-module convention and the
source file basename (confirm.svelte.ts).

Why this matters: vitest browser mode's @vitest/browser-playwright resolves
both spellings to the same module URL but registers a separate Playwright
route per spelling. The route-cleanup logic only unregisters the latest,
leaving an orphan that crashes the next session with
"[birpc] rpc is closed, cannot call resolveManualMock". Fixed upstream in
vitest PR #10267 (merged, not yet released). Normalising the spelling
removes the trigger from our side.

Refs: #553. Companion test-file changes follow in the next commit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/routes/admin/tags/[id]/TagDeleteGuard.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
 import { enhance } from '$app/forms';
 import { m } from '$lib/paraglide/messages.js';
-import { getConfirmService } from '$lib/shared/services/confirm.svelte.js';
+import { getConfirmService } from '$lib/shared/services/confirm.svelte';
 
 type FlatTag = {
 	id: string;