feat(sub): auto-detect subscription format by User-Agent (Updated) (#5826)

* feat(settings): add subscription format controls

* feat(sub): auto-detect subscription formats

* fix(xray): validate balancer regexes before save

* Revert "fix(xray): validate balancer regexes before save"

This reverts commit 8a208ce71b.

* doc(endpoints): align indent spaces

* doc(settings): improve error message formatting in validateSubUserAgentRegex

- Use NewErrorf with proper formatting instead of NewError with string concatenation
- Add comment explaining the rationale for returning original pattern value
- This preserves the intentional design where empty input is stored as empty
  in the DB and inherited as the runtime default at read time

---------

Co-authored-by: Tomilla <5007859+Tomilla@users.noreply.github.com>
Co-authored-by: Sanaei <ho3ein.sanaei@gmail.com>

frontend/src/pages/settings/SubscriptionFormatsTab.css

@@ -1,3 +1,18 @@
+.subscription-format-sections {
+  display: grid;
+  gap: 16px;
+}
+
+.subscription-format-card-title {
+  display: inline-flex;
+  align-items: center;
+  gap: 8px;
+}
+
+.subscription-format-card .ant-card-body {
+  padding: 0;
+}
+
 .format-settings {
   margin-bottom: 8px;
   border: 1px solid var(--ant-color-border-secondary);