fix(app): prevent infinite loop in @-mention scan when @ is at index 0

`findActiveFileMention` walked back through `@` candidates with
`lastIndexOf("@", atIndex - 1)`, but per spec a negative `position`
clamps to 0 — so once `atIndex` hit 0 with an invalid query (e.g. a
trailing space), the loop stepped from 0 back to 0 forever, freezing
the JS thread on every keystroke and forcing a quit.

Refs #596, #425

packages/app/src/utils/file-mention-autocomplete.test.ts

@@ -36,6 +36,14 @@ describe("findActiveFileMention", () => {
     });
     expect(mention).toBeNull();
   });
+
+  it("returns null when @ at start is followed by a delimiter", () => {
+    const mention = findActiveFileMention({
+      text: "@ ",
+      cursorIndex: 2,
+    });
+    expect(mention).toBeNull();
+  });
 });
 
 describe("applyFileMentionReplacement", () => {