fix: Open project dialog shows directories properly in web UI

[lxxonx]

Summary

Fixes #8325

The "Open project" dialog in the web UI shows "No folders found" instead of listing directories.

Changes

1. packages/opencode/src/server/server.ts:

   • Made query parameter optional with default ""
   • Added directory parameter to search from specified path
   • Added directory scanning logic when directory param is provided

2. packages/ui/src/hooks/use-filtered-list.tsx:

   • Fixed async items function handling with null safety (needle ?? "")

3. packages/app/src/components/dialog-select-directory.tsx:

   • Added null-safety: (filter ?? "").trim()

Verification

Tested locally by running opencode web and confirming the "Open project" dialog now shows directories from the home folder.