✨ feat(ratio-sync, ui): add built‑in “Official Ratio Preset” and harden upstream sync

Backend (controller/ratio_sync.go):
- Add built‑in official upstream to GetSyncableChannels (ID: -100, BaseURL: https://basellm.github.io)
- Support absolute endpoint URLs; otherwise join BaseURL + endpoint (defaults to /api/ratio_config)
- Harden HTTP client:
  - IPv4‑first with IPv6 fallback for github.io
  - Add ResponseHeaderTimeout
  - 3 attempts with exponential backoff (200/400/800ms)
- Validate Content-Type and limit response body to 10MB (safe decode via io.LimitReader)
- Robust parsing: support type1 ratio_config map and type2 pricing list
- Use net.SplitHostPort for host parsing
- Use float tolerance in differences comparison to avoid false mismatches
- Remove unused code (tryDirect) and improve warnings

Frontend:
- UpstreamRatioSync.jsx: auto-assign official endpoint to /llm-metadata/api/newapi/ratio_config-v1-base.json
- ChannelSelectorModal.jsx:
  - Pin the official source at the top of the list
  - Show a green “官方” tag next to the status
  - Refactor status renderer to accept the full record

Notes:
- Backward compatible; no API surface changes
- Official ratio_config reference: https://basellm.github.io/llm-metadata/api/newapi/ratio_config-v1-base.json

web/src/pages/Setting/Ratio/UpstreamRatioSync.jsx

@@ -151,8 +151,17 @@ export default function UpstreamRatioSync(props) {
         setChannelEndpoints((prev) => {
           const merged = { ...prev };
           transferData.forEach((channel) => {
-            if (!merged[channel.key]) {
-              merged[channel.key] = DEFAULT_ENDPOINT;
+            const id = channel.key;
+            const base = channel._originalData?.base_url || '';
+            const name = channel.label || '';
+            const isOfficial =
+              id === -100 ||
+              base === 'https://basellm.github.io' ||
+              name === '官方倍率预设';
+            if (!merged[id]) {
+              merged[id] = isOfficial
+                ? '/llm-metadata/api/newapi/ratio_config-v1-base.json'
+                : DEFAULT_ENDPOINT;
             }
           });
           return merged;