fix(FilePicker): reset filter string when navigating

[max65482]

Fixes nextcloud/calendar#8121

FilePicker:
The filter string is reset when navigating folders. This prevents unintended filtering.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.79%. Comparing base (b8e87a1) to head (0a4c18f).
⚠️ Report is 30 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2384   +/-   ##
=======================================
  Coverage   43.79%   43.79%           
=======================================
  Files          15       15           
  Lines         427      427           
  Branches       82       82           
=======================================
  Hits          187      187           
  Misses        211      211           
  Partials       29       29           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[susnux]

Thank you for the contribution, please fix the failing linter. Then this should be good.

[max65482]

Linter errors are fixed.