Feat : allow referencing files (#1648)

I implemented file referencing feature mentioned in issue #1591 

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Adds support for referencing app files in chat using @file:<path>. The
input now autocompletes files, and the backend validates and surfaces
referenced files in the chat context as read-only.

- **New Features**
  - Chat input autocompletes @ mentions for apps, prompts, and files.
- Recognizes @file:<path> and converts it to a structured mention on
submit.
- Backend parses @file mentions, checks file existence, and adds a
“Referenced Files” section to the system message.
- New get-app-files IPC handler and useAppFiles hook to load file paths
for the selected app (appFilesAtom added).
  - e2e test for mentioning a file and capturing server dump.

<!-- End of auto-generated description by cubic. -->

e2e-tests/mention_files.spec.ts

@@ -0,0 +1,15 @@
+import { test } from "./helpers/test_helper";
+
+test("mention file", async ({ po }) => {
+  await po.setUp({ autoApprove: true });
+
+  await po.importApp("minimal-with-ai-rules");
+  await po.goToAppsTab();
+  await po.getChatInput().click();
+  await po.getChatInput().fill("[dump] @");
+  await po.page.getByRole("menuitem", { name: "Choose AI_RULES.md" }).click();
+  await po.page.getByRole("button", { name: "Send message" }).click();
+  await po.waitForChatCompletion();
+
+  await po.snapshotServerDump("all-messages");
+});