feat: Add file manager view using VueFinder

Installs VueFinder and wires it to the backend /api/files endpoint with
JWT Bearer auth. Adds /files route, File Manager nav item (files.view
permission-gated, FolderOpen icon), and imports VueFinder CSS globally.
Driver token is computed reactively so it tracks token refreshes automatically.
Uses midnight theme to match the dark admin panel aesthetic.

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

frontend/src/views/admin/FileManagerView.vue

@@ -0,0 +1,49 @@
+<script setup lang="ts">
+import { computed } from 'vue'
+import { VueFinder, RemoteDriver } from 'vuefinder'
+import { useAuthStore } from '@/stores/auth'
+
+const auth = useAuthStore()
+
+// Recreate the RemoteDriver reactively so the token stays current across
+// automatic refresh cycles (useApi composable silently rotates accessToken).
+const driver = computed(
+  () =>
+    new RemoteDriver({
+      baseURL: '/api/files',
+      token: auth.accessToken ?? undefined,
+    })
+)
+
+// Non-persistent config passed to VueFinder per session.
+// maxFileSize in bytes — 10 MB limit matches the backend upload ceiling.
+const finderConfig = {
+  theme: 'midnight',
+  maxFileSize: 10 * 1024 * 1024,
+  showMenuBar: true,
+  showToolbar: true,
+}
+</script>
+
+<template>
+  <div class="space-y-6 p-6">
+    <div class="flex items-center justify-between">
+      <div>
+        <h1 class="text-2xl font-bold text-white">File Manager</h1>
+        <p class="text-sm text-gray-400 mt-1">Browse and edit your server files</p>
+      </div>
+    </div>
+
+    <div
+      class="bg-neutral-900 rounded-lg border border-neutral-800 overflow-hidden"
+      style="min-height: 640px;"
+    >
+      <VueFinder
+        id="corrosion-filemanager"
+        :driver="driver"
+        :config="finderConfig"
+        locale="en"
+      />
+    </div>
+  </div>
+</template>