Make User-Agent configurable (#3255)

Fixes #3253 

Adds a configuration option to overwrite the default `User-Agent` header
that is send at least by the calendar and news module. Allows other
modules to use the individual user agent as well.

The configuration accepts either a string or a function:
```
var config =
	{
		...
		userAgent: 'Mozilla/5.0 (My User Agent)',
		...
	}
```
or
```
var config =
	{
		...
		userAgent: () => 'Mozilla/5.0 (My User Agent)',
		...
	}
```

---------

Co-authored-by: Veeck <github@veeck.de>
Co-authored-by: veeck <gitkraken@veeck.de>
Co-authored-by: Karsten Hassel <hassel@gmx.de>
Co-authored-by: Kristjan ESPERANTO <35647502+KristjanESPERANTO@users.noreply.github.com>

CHANGELOG.md

@@ -13,6 +13,10 @@ planned for 2025-10-01
 
 Thanks to: @dathbe.
 
+### Added
+
+- Added configuration option for `User-Agent`, used by calendar & news module (#3255)
+
 ### Changed
 
 - [clock] Add CSS to prevent line breaking of sunset/sunrise time display (#3816)