feat: 侧边栏分组图标与导航样式优化

- 分组支持 icon 字段，可按名称自动匹配或手动选择
- 左侧导航与总览卡片显示彩色 SVG 图标
- 优化侧栏链接圆角与选中态样式

Co-authored-by: Cursor <cursoragent@cursor.com>

app.py

@@ -479,6 +479,7 @@ def create_app() -> Flask:
         if form.validate_on_submit():
             g = ServiceGroup(
                 name=form.name.data.strip(),
+                icon=(form.icon.data or "").strip(),
                 sort_order=form.sort_order.data or 0,
             )
             db.session.add(g)
@@ -497,6 +498,7 @@ def create_app() -> Flask:
         form = GroupForm(obj=g)
         if form.validate_on_submit():
             g.name = form.name.data.strip()
+            g.icon = (form.icon.data or "").strip()
             g.sort_order = form.sort_order.data or 0
             db.session.commit()
             flash("分组已更新", "success")
@@ -666,6 +668,17 @@ def _migrate_schema() -> None:
                     )
                 )
             print("[nav] 已为 services 表添加 embed_kind 列。", flush=True)
+        if "service_groups" in insp.get_table_names():
+            gcols = {c["name"] for c in insp.get_columns("service_groups")}
+            if "icon" not in gcols:
+                with db.engine.begin() as conn:
+                    conn.execute(
+                        text(
+                            "ALTER TABLE service_groups ADD COLUMN icon VARCHAR(16) "
+                            "NOT NULL DEFAULT ''"
+                        )
+                    )
+                print("[nav] 已为 service_groups 表添加 icon 列。", flush=True)
     except Exception as exc:
         print(f"[nav] 数据库结构迁移跳过或失败: {exc}", flush=True)