public IP silently disappears when ipv4.corecoding.com is unreachable (add fallback)

[dsalgadox]

Has this issue been covered in the Wiki?

• I have read the Wiki and my issue is not covered.

Is there an existing issue reported already?

• I have searched the existing issues, it does not exist.

What is the unexpected behavior?

Problem

_refreshIPAddress fetches the public IP from a single host
(https://ipv4.corecoding.com). When that host is down/unreachable, the
.catch(err => {}) swallows the error and the panel shows nothing — no value,
no flag, no hint. Several users reported the Public IP (WAN) disappearing on
Ubuntu 26.04 / GNOME Shell 50 over the last few days (the host has been
intermittent).

Steps to reproduce the unexpected behavior.

Fix

Try the project host first, then fall back to api.myip.com ({ip, cc},
keeps the country flag) and finally api.ipify.org (IP only). Behaviour is
unchanged when the primary host is up.

• Preserves ipv4.corecoding.com as the primary source and its response shape.
• Sources are tried in order; an empty IP / bad JSON / network error advances
  to the next provider.
• No new settings; no change to the panel/flag logic.

Relevant log output

### Testing
Verified each endpoint returns a usable response via the same Gio mechanism
the extension uses:

- `https://ipv4.corecoding.com` -> `{"IPv4":"...","countryCode":"BR"}`
- `https://api.myip.com`        -> `{"ip":"...","cc":"BR"}`
- `https://api.ipify.org?format=json` -> `{"ip":"..."}`

Confirmed the flag still resolves (type `network-br` -> `icon-br`).

What distribution and version of Linux are you using?

Distro: Ubuntu 26.04 — Kernel 7.0.0-22-generic

What version of Gnome are you using?

50.1