CrossRelay stage 0+1: Go gateway with Anthropic<->Chat Completions relay, Tauri shell, bilingual management UI

- Go gateway (stdlib only): /v1/messages non-stream + SSE, /v1/messages/count_tokens,
  management API /api/* (providers CRUD, apply/restore, settings, backups, logs)
- Protocol conversion: Anthropic Messages <-> Chat Completions with a streaming
  state machine keyed by content block index (text, tool_use, parallel calls)
- ~/.claude/settings.json guarded read/write: timestamped backups, managed env
  keys only, one-click restore
- Tauri 2 shell: spawns the gateway sidecar, parses the random port from stdout,
  stdin pipe watchdog prevents orphans, kills the child on exit
- React frontend: Dashboard/Providers/Settings, zh-CN + en-US, TanStack Query
- Tests: Go table-driven + httptest integration (all green), vitest + RTL

internal/gateway/logring.go

@@ -0,0 +1,34 @@
+package gateway
+
+import "sync"
+
+// logRing keeps the most recent log entries in memory.
+type logRing struct {
+	mu      sync.Mutex
+	entries []LogEntry
+	cap     int
+}
+
+func newLogRing(capacity int) *logRing {
+	return &logRing{cap: capacity}
+}
+
+func (r *logRing) add(entry LogEntry) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.entries = append(r.entries, entry)
+	if len(r.entries) > r.cap {
+		r.entries = r.entries[len(r.entries)-r.cap:]
+	}
+}
+
+// list returns entries newest first.
+func (r *logRing) list() []LogEntry {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	out := make([]LogEntry, len(r.entries))
+	for i, e := range r.entries {
+		out[len(r.entries)-1-i] = e
+	}
+	return out
+}