feat: worker lanes move to the System tab, in the roster's own idiom (4295)

Operator, 2026-09-23: *"where is the setting surface to set the number of
each kind of worker? we have a system view under settings but no options to
be set here"* — then *"move it to system tab and improve the view to be more
inline with other UI elements."*

They went looking under System and found a read-only roster. The card's own
first cut argued that a control had no place on a tab about things being
broken; that was wrong twice over. Half these lanes are optional and off by
DESIGN, so "not running" is the normal state someone comes here to change —
and a tab that only reports is a tab you leave in order to act somewhere else.

So it is the roster's second half now, and built out of the same parts: the
pane above says what is running, this one says how hard. Dot, name, detail,
numbers on the right; the controls take the place the roster gives to latency.
It was a bordered card wrapping a v-table, which is what made it read as
bolted on.

- The row styles move from SystemHealthTab's scoped block into app.css, so
  the second pane IS the first one's idiom rather than imitating it. A copy
  would have drifted the first time either was touched.
- Lane state uses the roster's four dots. A lane that is off reads grey, never
  red: colouring an intended state as a fault is how an indicator stops being
  read.
- The card owns its own polling. On Activity it rode that tab's 3s queue
  timer; System has none, so moving without this would have shown an empty
  pane to anyone who opened System first. 15s and `document.hidden`-guarded —
  it is a dial, not a live meter, and v-window keeps a hidden tab mounted.
- The table header carried the column names, so each control now labels
  itself: `slots · cap N`, `auto`, `on`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

frontend/src/components/settings/SystemActivityTab.vue

@@ -17,11 +17,6 @@
       </v-card-text>
     </v-card>
 
-    <!-- The dial for the pane above (milestone 422). Directly beneath it
-         because the two are read together: you see a queue growing and give
-         that lane more slots without leaving the pane. -->
-    <WorkerLanesCard />
-
     <!-- The non-Celery halves of the app (2026-07-02): the GPU agent does the
          majority of processing and downloads feed the library — Activity is
          the whole-app pulse, not just the worker queues. -->
@@ -177,7 +172,6 @@ import { useSystemActivityStore } from '../../stores/systemActivity.js'
 import { formatRelative as fmtRelative } from '../../utils/date.js'
 import ErrorDetailModal from '../common/ErrorDetailModal.vue'
 import QueuesTable from './QueuesTable.vue'
-import WorkerLanesCard from './WorkerLanesCard.vue'
 import CardHeading from '../common/CardHeading.vue'
 import GpuActivityPanel from './GpuActivityPanel.vue'
 import DownloadsActivityPanel from './DownloadsActivityPanel.vue'
@@ -242,7 +236,6 @@ function pollQueues() {
   store.loadQueues()
   store.loadWorkers()
   store.loadRecentRuns()
-  store.loadLanes()
 }
 function pollFailures() {
   if (document.hidden) return