luci-app-ddns: show the URL which will be used.

[adelton]

• This PR is not from my main or master branch 💩, but a separate branch ✅
• Each commit has a valid ✒️ Signed-off-by: <[email protected]> row (via git commit --signoff)
• Each commit and PR title has a valid 📝 <package name>: title first line subject for packages
• Incremented 🆙 any PKG_VERSION in the Makefile
• Tested on: x86_64 VM with OpenWrt 24.10.0-rc5 (r28304-6dacba30a7)
• ( Preferred ) Mention: @ the original code author for feedback
• ( Preferred ) Screenshot or mp4 of changes:
  
• ( Optional ) Closes luci-app-ddns: Namecheap DDNS setting are misleading #2436
• ( Optional ) Depends on: e.g. openwrt/packages#pr-number in sister repo
• Description: When using the DDNS settings, I was always puzzled a bit by the Replaces [DOMAIN] in Update-URL (URL-encoded) comments. It's certainly nice to have that information but I always wondered "what is that Update-URL?" for this service? Then I came across luci-app-ddns: Namecheap DDNS setting are misleading #2436 which basically discusses that this DDNS provider does not use username and expects host (?) instead. We likely cannot finetune the description of the fields in the UI to this case but maybe if we actually showed the admin the Update-URL which will be used, it might be easier for them to figure out which field to use for what.

[adelton]

This can also be considered as a fix for #5297.

[adelton]

In #3582 folks suggest that LuCI should parse the URL for the [FIELD]s to then determine what UI fields should be mandatory.

If we don't plan to do that (or do that right now) and we start to show the URL in the UI with this PR, could we make all the currently-mandatory fields optional? And basically make it the responsibility of the user to fill in the fields used in the URL, based on the hints in under the fields?