feat: add growth tracking page and related functionality

- Implemented a new Growth page to track practice time and trends.
- Added API integration for fetching review data.
- Created components for displaying practice statistics and trends.
- Updated navigation titles for the main index and growth pages.
- Removed unused styles from the index page.
- Introduced a Projects management page for adding and editing practice projects.
- Developed a Record form for logging practice sessions with validation.
- Added utility functions for date manipulation and duration formatting.
- Implemented error handling and session management in the practice service.
- Created unit tests for the practice service to ensure reliability.

backend/internal/identity/wechat.go

@@ -0,0 +1,61 @@
+package identity
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"io"
+	"net/http"
+	"net/url"
+	"time"
+)
+
+var ErrInvalidCode = errors.New("WeChat login code is invalid or expired")
+var errUnavailable = errors.New("WeChat identity verification is unavailable")
+
+// WeChat exchanges wx.login codes at the server only. Client is an optional
+// network dependency for tests; production uses a bounded HTTPS client.
+type WeChat struct {
+	AppID     string
+	AppSecret string
+	Client    *http.Client
+}
+
+// Exchange returns an app-scoped openid after WeChat verifies the code. It never
+// returns session_key or wraps URL errors, since exchange URLs contain secrets.
+func (w *WeChat) Exchange(ctx context.Context, code string) (string, error) {
+	if w.AppID == "" || w.AppSecret == "" {
+		return "", errUnavailable
+	}
+	query := url.Values{"appid": {w.AppID}, "secret": {w.AppSecret}, "js_code": {code}, "grant_type": {"authorization_code"}}
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, "https://api.weixin.qq.com/sns/jscode2session?"+query.Encode(), nil)
+	if err != nil {
+		return "", errUnavailable
+	}
+	client := w.Client
+	if client == nil {
+		client = &http.Client{Timeout: 5 * time.Second, CheckRedirect: func(*http.Request, []*http.Request) error { return http.ErrUseLastResponse }}
+	}
+	response, err := client.Do(request)
+	if err != nil {
+		return "", errUnavailable
+	}
+	defer response.Body.Close()
+	if response.StatusCode != 200 {
+		return "", errUnavailable
+	}
+	var result struct {
+		OpenID  string `json:"openid"`
+		ErrCode int    `json:"errcode"`
+	}
+	if err = json.NewDecoder(io.LimitReader(response.Body, 16*1024)).Decode(&result); err != nil {
+		return "", errUnavailable
+	}
+	if result.ErrCode == 40029 || result.ErrCode == 40163 {
+		return "", ErrInvalidCode
+	}
+	if result.ErrCode != 0 || result.OpenID == "" {
+		return "", errUnavailable
+	}
+	return w.AppID + ":" + result.OpenID, nil
+}