fix(ops): prevent monitoring trend cards from growing unbounded

The concurrency / switch-rate / throughput cards on the ops dashboard
sit in grid cells that only set `min-h-[360px]` (no definite height).
Their inner card uses `h-full`, which resolves to `auto` when the parent
height is `auto`. Combined with the Chart.js `responsive` +
`maintainAspectRatio: false` charts, this forms a height feedback loop:
the canvas reads the parent height to size itself, the content then grows,
the next ResizeObserver tick reads an even larger height, and the cards
stretch downward without bound.

On wide screens (`lg:grid-cols-4`) a sibling card usually fixes the row
height via `align-items: stretch`, masking the issue. It surfaces when no
sibling bounds the row height — e.g. the single-column (`grid-cols-1`)
stacked layout on narrow viewports, or when the concurrency card collapses
to little content. `min-h` only sets a floor, not a ceiling.

Fix: give the two Chart.js canvas cells a definite height (`h-[360px]`) so
the responsive resize has a fixed reference and the loop cannot run. The
concurrency card is not a responsive canvas, so it keeps `min-h-[360px]`
to avoid clipping its content.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/views/admin/ops/OpsDashboard.vue

@@ -44,7 +44,7 @@
         <div class="lg:col-span-1 min-h-[360px]">
           <OpsConcurrencyCard :platform-filter="platform" :group-id-filter="groupId" :refresh-token="dashboardRefreshToken" />
         </div>
-        <div class="lg:col-span-1 min-h-[360px]">
+        <div class="lg:col-span-1 h-[360px]">
           <OpsSwitchRateTrendChart
             :points="switchTrend?.points ?? []"
             :loading="loadingSwitchTrend"
@@ -52,7 +52,7 @@
             :fullscreen="isFullscreen"
           />
         </div>
-        <div class="lg:col-span-2 min-h-[360px]">
+        <div class="lg:col-span-2 h-[360px]">
           <OpsThroughputTrendChart
             :points="throughputTrend?.points ?? []"
             :by-platform="throughputTrend?.by_platform ?? []"