feat(plugins): 4 个业务插件代码 + 前台公开页面改造

plugins/navManager：导航 CRUD + 导航分类 CRUD（独立 blueprint）
plugins/friendManager：友情链接 CRUD
plugins/aboutManager：关于本站 CRUD
plugins/siteStats：访问统计主页 + JSON 接口 + /site/* GET 埋点 + nav 卡片点击埋点

applications/view/public/nav.py：导航聚合页 / 分类详情 / JSON 接口，支持 visit_count 注入
applications/view/public/about.py：关于本站公开页
applications/view/public/friend.py：友情链接公开页

templates/public/base.html：响应式首页（搜索框 / 分类抽屉 / 5 主题切换 / footer 隐私小字）
templates/public/index.html / category.html：卡片右下角访问次数
templates/public/about.html / friend.html：关于 + 友链前台页

docs/plugins-development.md：插件开发完整指南（生命周期 + 4 个示例 + FAQ + framework 迁移）

scripts/：探针与端到端验证脚本（probe_*/verify_*/test_*）

scripts/verify_beacon_click.py

@@ -0,0 +1,30 @@
+"""验证点击卡片触发 sendBeacon，数据库 +1。"""
+from playwright.sync_api import sync_playwright
+from app import app
+from applications.extensions import db
+from applications.models import NavClick
+
+with app.app_context():
+    before = db.session.query(db.func.count(NavClick.id)).filter(NavClick.nav_id == 1).scalar()
+print('before click:', before)
+
+with sync_playwright() as p:
+    browser = p.chromium.launch()
+    ctx = browser.new_context(viewport={'width': 1280, 'height': 800})
+    page = ctx.new_page()
+    page.goto('http://127.0.0.1:5000/site/', wait_until='networkidle')
+    # 拦截请求，看是不是 POST
+    reqs = []
+    page.on('request', lambda r: reqs.append(r) if '/site/nav/1/click' in r.url else None)
+    # 点击第一张卡片
+    card = page.locator('.card[data-nav-id="1"]').first
+    card.click(modifiers=['Meta'])  # 按住 Meta 打开新标签，保持当前页
+    page.wait_for_timeout(500)
+    print('captured requests:', [(r.method, r.url) for r in reqs])
+    browser.close()
+
+with app.app_context():
+    after = db.session.query(db.func.count(NavClick.id)).filter(NavClick.nav_id == 1).scalar()
+print('after click:', after)
+assert after == before + 1, f"Expected {before+1}, got {after}"
+print('PASS: beacon recorded click')