Retain host and server scope during investigations

src/routes/capacity/+page.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+    import BackLink from '$ui/components/BackLink.svelte';
     import { contextHref } from '$lib/navigation-context.js';
     import { onMount, untrack } from 'svelte';
     import { beforeNavigate, afterNavigate, goto } from '$app/navigation';
@@ -36,7 +37,12 @@
         if (query.hostId) params.set('host', query.hostId);
         return params;
     }
-    function scopeUrl(change: Partial<CapacityQuery>) { return `/capacity?${queryParams({ ...report.query, ...change })}`; }
+    function scopeUrl(change: Partial<CapacityQuery>) {
+        const params = queryParams({ ...report.query, ...change });
+        const returnTo = page.url.searchParams.get('returnTo');
+        if (returnTo) params.set('returnTo', returnTo);
+        return `/capacity?${params}`;
+    }
     function cancel() { sequence++; request?.abort(); request = null; refreshing = false; }
     async function refresh(signal?: AbortSignal) {
         if (navigating || refreshing) return;
@@ -72,6 +78,7 @@
     <SectionHeader title="Capacity" subtitle="See what your hosts can run, what is limiting them, and how headroom changes.">
         {#snippet actions()}<Button href="/hosts">Manage hosts</Button><Button disabled={refreshing || navigating} onclick={() => void refresh()}>{refreshing ? 'Refreshing…' : 'Refresh'}</Button>{/snippet}
     </SectionHeader>
+    {#if page.url.searchParams.has('returnTo')}<BackLink fallback="/hosts" />{/if}
     {#if refreshError || navigationError}<div class="refresh-error" role="alert"><strong>{navigationError || refreshError}</strong><span>Showing the last successfully loaded capacity report.</span></div>{/if}
     <div class="toolbar">
         <div class="filters"><Field label="Host"><Select value={report.query.hostId ?? ''} options={hostOptions} disabled={navigating} onchange={event => void changeScope({ hostId: (event.currentTarget as HTMLSelectElement).value || null })} /></Field>