applets/notification: improve DND mode explanatory text

This text suffered from two issues:

• By trying to generate an English sentence, it became a string puzzle because not all possible relative date strings would work after the word "until". For example, "Until In two days" would be wrong.
• It used inappropriate capitalization of the date despite trying to be in a sentence.

This commit fixes both issues by splitting the two parts of the text on either side of a colon, creating a logical break between them.

BUG: 421989 FIXED-IN: 5.27

cc @teams/vdg

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