refactor(nav): replace static back-link hrefs with BackButton

All 7 in-scope back navigation links converted to use history.back().
Admin panel mobile chevron converted inline (icon-only, different
visual pattern). Cancel buttons left as static <a> links.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/admin/users/[id]/+page.svelte

@@ -23,9 +23,11 @@ $effect(() => {
 <div class="flex flex-1 flex-col overflow-hidden">
 	<!-- Detail panel header -->
 	<div class="flex items-center border-b border-line px-5 py-3">
-		<a
-			href="/admin/users"
-			class="mr-3 inline-flex items-center gap-1 text-xs text-ink-3 hover:text-ink md:hidden"
+		<button
+			type="button"
+			onclick={() => history.back()}
+			aria-label={m.btn_back()}
+			class="mr-3 inline-flex items-center gap-1 text-xs text-ink-3 outline-none hover:text-ink focus-visible:ring-2 focus-visible:ring-focus-ring md:hidden"
 		>
 			<svg
 				class="h-4 w-4"
@@ -37,7 +39,7 @@ $effect(() => {
 			>
 				<path stroke-linecap="round" stroke-linejoin="round" d="M15 19l-7-7 7-7" />
 			</svg>
-		</a>
+		</button>
 		<h2 class="flex-1 font-sans text-sm font-bold text-ink">
 			{m.admin_user_edit_heading({ username: data.editUser.email })}
 		</h2>