refactor: improve testability across app, ip, dns, notify, config

[orvice]

• App.Run now uses a Ticker, returns error instead of os.Exit, and exposes WithInterval for tests; updateIP split into findRecord / appendRecord / updateRecord helpers.
• IfconfigCo accepts an injectable HTTP client and URL via the new NewIfconfigCo constructor; production path unchanged.
• dns.New returns (LibDNS, error) so unknown providers fail loudly; empty value still defaults to Cloudflare.
• notify drops the package-level notifiers slice / Init / AddNotifier / Notify globals. TelegramNotifier now depends on a minimal tgSender interface; new MultiNotifier replaces the global fan-out.
• config.New uses an independent viper.New() instance with explicit BindEnv calls so tests can run without sharing global viper state.
• main.go propagates Run errors and exits non-zero on failure.
• Added unit tests with hand-written fakes for app, ip (httptest), dns provider selection, and notify (fake bot + MultiNotifier).