dns/ddclient: Improve Cloudflare configuration UX and help text

[bks22]

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Is your feature request related to a problem? Please describe.
Configuring Dynamic DNS with Cloudflare API Tokens is unintuitive and leads to confusing errors with no guidance on how to resolve them.

1. API Token authentication requires token as username. When using a Cloudflare API Token (not Global API Key), the Username field must be set to the literal string token. Using an email address results in Invalid format for X-Auth-Key header, which doesn't indicate that the username field needs to be changed.

2. Hostname requires FQDN despite Zone being configured. Even when a Zone is configured (e.g., example.com), the Hostname field requires the full FQDN (e.g., host.example.com), not just the host portion (host). Using just the hostname results in No 'A' record at Cloudflare, which is misleading because the record exists — ddclient just can't find it due to the naming mismatch.

Describe the solution you'd like

1. Add placeholder or help text to the Username field: "token" for API Tokens, or email for Global API Key
2. Add help text to the Hostname field: Full domain name required (e.g., host.example.com)

Describe alternatives you've considered

• Auto-append the zone to the hostname when only the host portion is provided
• Validate the configuration by making a test API call when saving, to catch misconfigurations early

Additional context

• ddclient's Cloudflare protocol uses login=token to trigger Bearer authentication
• Previous issues around Cloudflare API token support (os-ddclient with opnsense backend and cloudflare doesn't work with API token #3427, os-ddclient Cloudflare API Key #2842) have been resolved, but the UI still doesn't guide users toward the correct configuration
• Cloudflare's own API Token docs don't explain ddclient integration, so users rely entirely on OPNsense's UI for guidance

[AdSchellevis]

https://docs.opnsense.org/manual/dynamic_dns.html#cloudflare ?