feat: agent avatars, task tags, search, edit/delete agents, cron readability, timezone fix

- Agent avatars: first-letter + deterministic color avatar in all views
- Agent management: edit (name/description) and delete with confirmation
- Execution API: validates agent existence, returns 410 if deleted
- Task tags: JSON array field, editable with tag pills UI
- Task search: by name (?q=) and by tags (?tags=) and by status
- Task edit: simplified to name/tags/description only (not agent params)
- Cron display: human-readable Chinese (e.g. '每5分钟', '每天 09:00')
- Timezone fix: UTC→local via dayjs.utc().local() across all pages
- Content area: full width (removed max-width:1200px constraints)
- Login page: standalone layout without sidebar

backend/app/models/task.py

@@ -18,6 +18,7 @@ class ScheduledTask(Base):
     cron_expression: Mapped[str] = mapped_column(String(64), nullable=False, comment="Cron 表达式, e.g. */5 * * * *")
     grace_period: Mapped[int] = mapped_column(Integer, default=300, comment="容忍秒数, 超时未执行则告警")
     status: Mapped[str] = mapped_column(String(32), default="active", comment="active / paused / stopped")
+    tags: Mapped[str | None] = mapped_column(Text, nullable=True, comment="标签 JSON 数组, e.g. [\"数据同步\",\"重要\"]")
 
     # 冗余字段方便查询
     last_run_at: Mapped[datetime | None] = mapped_column(DateTime, nullable=True, comment="最近一次执行时间")