maple-agent: adopt a task's locked model in the selector

[maple-engineering]

Fixes #1050.

The runtime already locks a task to its first model. The desktop composer kept a single global last-used selection, so switching from a GLM task to a Kimi task still showed GLM and send hit the lock error.

This matches ACP: once a task has messages, the selector shows that model and the menu only offers it. The account default is unchanged, so New Task restores it. Picking a model on a draft still updates the default.

Follow-up (ed292e4):

• The lock now follows the task on screen. A task you just started locks as soon as its first message lands; before, it stayed unlocked until you reopened it.
• Leaving a task ends its lock. Before, archiving or deleting the last task in a project left the empty screen on that task's model, and a new task started on it.
• A locked chip shows a lock icon. Its menu row says "Locked to this task. Start a new task to switch."

Test plan

• cargo test -p maple-agent-app --locked ui::chat::tests
• cargo clippy -p maple-agent-app --all-targets --locked -- -D warnings
• pre-commit: fmt, clippy, workspace tests
• just ci (474 crate + 418 app tests). The two new tests fail on a5139f7.
• Local stack (OpenSecret + billing, Pro fixture), real app:
  • The first message on a new glm-5-3 task locks the chip at once.
  • A New Task draft offers every model.
  • A kimi-k3 task locks to kimi-k3. Switching back to the GLM task shows glm-5-3, and sending works.
  • /model kimi on the GLM task shows the lock notice.
  • Archiving the last task in the project leaves an unlocked glm-5-3 with the full menu.

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