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/alembic/versions/0001_initial.py

@@ -48,6 +48,7 @@ def upgrade() -> None:
         sa.Column("cron_expression", sa.String(64), nullable=False),
         sa.Column("grace_period", sa.Integer, default=300),
         sa.Column("status", sa.String(32), default="active"),
+        sa.Column("tags", sa.Text, nullable=True),
         sa.Column("last_run_at", sa.DateTime, nullable=True),
         sa.Column("last_run_result", sa.String(32), nullable=True),
         sa.Column("next_run_at", sa.DateTime, nullable=True),