feat(server): config file editor — read/edit/save a host config file per instance

The Server page's config-honesty note now leads somewhere real: a
Configuration file panel that loads a config file from the instance
(prefilled with the game's primaryConfigFile hint — server.cfg,
ServerSettings.ini, GameXishu.json), edits it in a mono textarea, and
saves it straight to the host through the jailed agent file bridge.
Not-found is handled gracefully (empty editor to create). Works across
games; gameProfiles gains primaryConfigFile per game.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/stores/instances.ts

@@ -106,6 +106,23 @@ export const useInstancesStore = defineStore('instances', () => {
     return api.post<Record<string, unknown>>(`/instances/${id}/rcon`, { command })
   }
 
+  /** Read a config/text file from the current instance (jailed to its root). */
+  async function readFile(path: string): Promise<string> {
+    const id = currentId.value
+    if (!id) throw new Error('No instance selected')
+    const res = await api.get<{ content?: string }>(
+      `/instances/${id}/file?path=${encodeURIComponent(path)}`,
+    )
+    return res?.content ?? ''
+  }
+
+  /** Write a config/text file to the current instance. */
+  async function writeFile(path: string, content: string): Promise<void> {
+    const id = currentId.value
+    if (!id) throw new Error('No instance selected')
+    await api.put(`/instances/${id}/file`, { path, content })
+  }
+
   return {
     instances,
     currentId,
@@ -117,5 +134,7 @@ export const useInstancesStore = defineStore('instances', () => {
     select,
     lifecycle,
     rcon,
+    readFile,
+    writeFile,
   }
 })