fix(sub): default https:// for scheme-less support and profile URLs

A support URL saved without a scheme (e.g. "t.me/handle") is served
verbatim in the subscription Support-Url header and page data, and client
apps resolve it relative to the subscription domain — clicking it lands
on "https://panel.example/t.me/handle". Same hazard for the profile URL.

Default the scheme to https:// when none is present, both when saving the
settings and when reading already-stored values, so existing databases are
covered without a migration. Deliberate non-http schemes (tg://, mailto:,
tel:) pass through untouched, which is why these two fields don't go
through SanitizeHTTPURL's http(s)-only validation.

Closes #5738

internal/util/common/url.go

@@ -0,0 +1,21 @@
+package common
+
+import "strings"
+
+// EnsureURLScheme prepends https:// to a URL that carries no scheme, so
+// subscription apps and browsers don't resolve it relative to the panel's own
+// domain (e.g. "t.me/support" turning into "https://panel.example/t.me/support").
+// Values with an explicit scheme (https://, tg://, mailto:, tel:) and empty
+// strings pass through untouched.
+func EnsureURLScheme(raw string) string {
+	trimmed := strings.TrimSpace(raw)
+	if trimmed == "" {
+		return ""
+	}
+	if strings.Contains(trimmed, "://") ||
+		strings.HasPrefix(trimmed, "mailto:") ||
+		strings.HasPrefix(trimmed, "tel:") {
+		return trimmed
+	}
+	return "https://" + trimmed
+}