feat(webui): the scheduled-task panel on top of ADR 0012's store

[Andre-1998]

PR #21 shipped the backend for Module L: its own store at <dataDir>/webui/scheduled-tasks.sqlite and an in-process tick beside the heartbeat in WebuiService. The user-visible half was never built — the 定时 rail row was still rendered inert and the client had no scheduled-task component at all. This adds it, against the six operations PR #21 registered.

The panel's 创建 dropdown offers 手动创建 and 在对话中创建. The manual dialog carries the fields the store actually has — name, agent, prompt, run mode, and a schedule that is either 仅一次 or 每 N 分钟. The conversational path opens a new session, sends a guiding message, and creates the task when the user says they are done. The list groups by agent and shows last/next run with the last result; a slot missed while no host was running surfaces as missed with its count rather than being presented as "it never existed".

Two things this deliberately does not do. It does not invent a listAgents operation — the agent choices are derived from the session listing the client already has, so no server file changes and nothing collides with merged work. And it does not draw controls the store cannot persist: 项目, 模型, and calendar periods (每天几点 / 每周几) have no columns, so the dialog says so in one line rather than shipping a disabled control that silently drops the value. That gap is a storage extension, not a UI decision.

Client-only by construction: no src/server/**, no local-runtime*, no docs/**, no dependency change. The panel opens on getScheduledTaskCapability and reports reason when the capability is unavailable, so an absent scheduler is a message rather than an empty screen.

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