Activity: one stream at a time, none while hidden; the header by the chat's width

The events stream is held while it opens, so a reconnect and the window
showing again cannot open two; while the window is hidden in the tray it is
not reopened (the server polls for each), and showing the window opens it
again. The Thinking switch says 'Think before answering' in the header too:
'Thinking' is the catalogues' word for a reply in progress. The chat's header
restacks by the chat column's own width (a container query at --bp-narrow),
so a chat beside the activity panel keeps its title.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ui/js/chat.js

@@ -832,8 +832,6 @@ export function newChat(kind) {
   const showLevels = (id) => {
     const model = models.find((m) => m.id === id)
     const control = effortControl({ model, value: levelsOf(model).start, id: "new-effort" })
-    const switchOnly = levelsOf(model).control === "switch"
-    effortField.children[0].textContent = switchOnly ? t("Thinking") : t("Reasoning effort")
     effortField.children[0].htmlFor = "new-effort"
     effortField.children[1].replaceWith(control || h("span", { id: "new-effort" }))
     effortField.hidden = !control