feat(oauth): add remote OAuth callback support with session management

Introduce a centralized OAuth session store with TTL-based expiration
  to replace the previous simple map-based status tracking. Add a new
  /api/oauth/callback endpoint that allows remote clients to relay OAuth
  callback data back to the CLI proxy, enabling OAuth flows when the
  callback cannot reach the local machine directly.

  - Add oauth_sessions.go with thread-safe session store and validation
  - Add oauth_callback.go with POST handler for remote callback relay
  - Refactor auth_files.go to use new session management APIs
  - Register new callback route in server.go

internal/api/handlers/management/oauth_callback.go

@@ -0,0 +1,100 @@
+package management
+
+import (
+	"errors"
+	"net/http"
+	"net/url"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+)
+
+type oauthCallbackRequest struct {
+	Provider    string `json:"provider"`
+	RedirectURL string `json:"redirect_url"`
+	Code        string `json:"code"`
+	State       string `json:"state"`
+	Error       string `json:"error"`
+}
+
+func (h *Handler) PostOAuthCallback(c *gin.Context) {
+	if h == nil || h.cfg == nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"status": "error", "error": "handler not initialized"})
+		return
+	}
+
+	var req oauthCallbackRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"status": "error", "error": "invalid body"})
+		return
+	}
+
+	canonicalProvider, err := NormalizeOAuthProvider(req.Provider)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"status": "error", "error": "unsupported provider"})
+		return
+	}
+
+	state := strings.TrimSpace(req.State)
+	code := strings.TrimSpace(req.Code)
+	errMsg := strings.TrimSpace(req.Error)
+
+	if rawRedirect := strings.TrimSpace(req.RedirectURL); rawRedirect != "" {
+		u, errParse := url.Parse(rawRedirect)
+		if errParse != nil {
+			c.JSON(http.StatusBadRequest, gin.H{"status": "error", "error": "invalid redirect_url"})
+			return
+		}
+		q := u.Query()
+		if state == "" {
+			state = strings.TrimSpace(q.Get("state"))
+		}
+		if code == "" {
+			code = strings.TrimSpace(q.Get("code"))
+		}
+		if errMsg == "" {
+			errMsg = strings.TrimSpace(q.Get("error"))
+			if errMsg == "" {
+				errMsg = strings.TrimSpace(q.Get("error_description"))
+			}
+		}
+	}
+
+	if state == "" {
+		c.JSON(http.StatusBadRequest, gin.H{"status": "error", "error": "state is required"})
+		return
+	}
+	if err := ValidateOAuthState(state); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"status": "error", "error": "invalid state"})
+		return
+	}
+	if code == "" && errMsg == "" {
+		c.JSON(http.StatusBadRequest, gin.H{"status": "error", "error": "code or error is required"})
+		return
+	}
+
+	sessionProvider, sessionStatus, ok := GetOAuthSession(state)
+	if !ok {
+		c.JSON(http.StatusNotFound, gin.H{"status": "error", "error": "unknown or expired state"})
+		return
+	}
+	if sessionStatus != "" {
+		c.JSON(http.StatusConflict, gin.H{"status": "error", "error": "oauth flow is not pending"})
+		return
+	}
+	if !strings.EqualFold(sessionProvider, canonicalProvider) {
+		c.JSON(http.StatusBadRequest, gin.H{"status": "error", "error": "provider does not match state"})
+		return
+	}
+
+	if _, errWrite := WriteOAuthCallbackFileForPendingSession(h.cfg.AuthDir, canonicalProvider, state, code, errMsg); errWrite != nil {
+		if errors.Is(errWrite, errOAuthSessionNotPending) {
+			c.JSON(http.StatusConflict, gin.H{"status": "error", "error": "oauth flow is not pending"})
+			return
+		}
+		c.JSON(http.StatusInternalServerError, gin.H{"status": "error", "error": "failed to persist oauth callback"})
+		return
+	}
+
+	c.JSON(http.StatusOK, gin.H{"status": "ok"})
+}