fix(desktop): correct @ file search replacement in EmptySession

Same bug as ChatInput — inserting filename at cursor without removing
the @filter trigger text, producing @foofile.ts instead of file.ts.

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

cc-haha/desktop/src/pages/EmptySession.tsx

@@ -484,8 +484,9 @@ export function EmptySession() {
                 filter={atFilter}
                 onSelect={(_path, name) => {
                   if (atCursorPos >= 0) {
-                    const newValue = `${input.slice(0, atCursorPos)}${name}${input.slice(atCursorPos)}`
-                    const newCursorPos = atCursorPos + name.length
+                    const atStart = atCursorPos - atFilter.length - 1
+                    const newValue = `${input.slice(0, atStart)}${name}${input.slice(atCursorPos)}`
+                    const newCursorPos = atStart + name.length
                     setInput(newValue)
                     setFileSearchOpen(false)
                     setAtFilter('')