Tweak strings related to recorder docker

This commit basically adjusts various strings related to the recorder docker so that they read better.

The two most notable changes:

• I made it so that the video duration in the export UI always shows two digit milliseconds, as I thought it looks weird otherwise.
• I also made it so that the buttons in the export UI for browsing and exporting the recording give less technical warning windows if the user did not record any frames yet.

Since these string changes are arguably nitpicks for the most part, it isn't necessary to backport them to 5.0.

Test Plan

Observe various changed strings. Make sure the added logic for the warnings didn't break anything by attempting to export with and without frames.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.