#3550 [GUI] add Load and Unload to the chat header

本文

Problem and result: The monitor page posts `/load` and `/unload`. The chat page had no way to do that. The chat header now posts the same two routes. Metrics stay unloaded until the engine is ready, so the button follows a load, a reply, or an image job started on this page until a sample agrees. A chat reply stays ahead of an image request on this page. A confirmed unload stays an unload if a poll arrives during the dialog.

A narrow window scrolls the tab strip and keeps Unload and the theme button in view. The pill text hides only when the header row overflows.

Related discussion: none. This uses the load and unload routes the monitor page already has.

Approach: One header button calls those routes. The label comes from the latest metrics sample. Until that sample arrives, the job started on this page sets the label. The header itself does not scroll. The tab strip does. From 458px down, Unload and the tab strip use a smaller minimum width. From 383px down, the brand is hidden. `fitHeader()` drops the pill text only when the row's `scrollWidth` is greater than its `clientWidth`.

Validation:
  Passed: `node --check serve/web/app.js` on `7c7fe569`. Firefox 158, headless, `--window-size`, on Linux: at 320, 361, 373, 374, 383, 401, 456, 458, and 720, Unload and the theme button were inside the header, and the header's `clientWidth` equalled its `scrollWidth`. At 720 with the Images tab and the long pill, About scrolls inside the tab strip and the long pill stays.
    The author then used the running chat page on two Linux machines and reports that it functions as intended. The desktop serves with the HIP build on an RX 6800 and an RX 6900 XT. The laptop serves with the CUDA and HIP hybrid build on an RTX 4060 and a Radeon 890M. No command log was attached to that report.
  Failed: none observed.
  Not run: a written click-through of the image-queue toast and the button title when a chat and an image are both waiting.

Limits: An image that is only queued can still raise the "stop the reply" toast when no image job is marked busy. When a local chat and a local image are both waiting in the unloaded window, the button title uses the image sentence.