fix(stage-6): dashboard hardening + audit Lows cherry-pick

Closes 4 dashboard hardening findings and 5 of the Lows from the audit.

Dashboard hardening:
- Subtabs no longer re-fetch the app via api.apps.get on every page
  load. users/files/dead-letters drop the fetch outright (the variable
  was set but never read); queues + queues/[name] now consume the
  layout's AppContext via getContext for the page title. The layout's
  reloadApp() owns the historical-slug redirect — subtab-local redirect
  blocks are removed so there's no race.
- The global :global(details > summary::before) chevron is now scoped
  to details.chevron. The script editor's "Advanced sandbox" details
  and the inbound-email-shape help-text both opt in; the script
  exec-list logs no longer inherit a spurious chevron.
- deriveTab now matches the path segment by anchored ===, so a future
  /apps/<slug>/queues-archived route wouldn't activate the queues tab.

Lows cherry-pick:
- ExecError gains Serialize/Deserialize derives + a snake_case tag so
  RemoteExecutorClient (cluster mode v1.3+) can round-trip the variant.
- triggers_api rejects queue triggers whose visibility_timeout_secs is
  below the dispatcher's per-message executor budget; with no minimum
  the reclaim task races the handler and the queue silently
  double-delivers. Existing test using 5s updated to 30s.
- New migration 0040: execution_logs.script_id cascade switched from
  ON DELETE CASCADE to ON DELETE SET NULL so deleting a script no
  longer wipes the forensic history that motivated the delete.
- New migration 0041: dead_letters composite index on
  (app_id, created_at DESC) so the "list all" dashboard view stops
  falling back to seqscan + sort when unresolved=false.
- Schema snapshot re-blessed.

Deferred to v1.2: the ExecRequest principal serde(skip) marker
(documented in-place; the cluster-mode PR will introduce the wire-safe
snapshot at that point) and the `pic --help` mention of
`picloud admin reset-password` (one-line follow-up).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dashboard/src/routes/apps/[slug]/queues/+page.svelte

@@ -1,13 +1,17 @@
 <script lang="ts">
 	import { base } from '$app/paths';
-	import { goto } from '$app/navigation';
 	import { page } from '$app/state';
-	import { api, ApiError, type App, type QueueSummary } from '$lib/api';
-
-	import { onDestroy } from 'svelte';
+	import { getContext, onDestroy } from 'svelte';
+	import { api, ApiError, type QueueSummary } from '$lib/api';
+	import { APP_CTX_KEY, type AppContext } from '../+layout.svelte';
 
 	let slug = $derived(page.params.slug ?? '');
-	let app = $state<App | null>(null);
+	// Read the layout's app store instead of re-fetching. The layout
+	// already drives the historical-slug redirect, so we don't repeat
+	// that here either.
+	const ctx = getContext<AppContext>(APP_CTX_KEY);
+	const appStore = ctx?.app;
+	let appName = $derived($appStore?.name ?? slug);
 	let queues = $state<QueueSummary[]>([]);
 	let loading = $state(false);
 	let error = $state<string | null>(null);
@@ -16,22 +20,6 @@
 	let autoRefresh = $state(false);
 	let autoRefreshHandle: ReturnType<typeof setInterval> | null = null;
 
-	async function loadApp() {
-		try {
-			const fetched = await api.apps.get(slug);
-			// Mirror apps/[slug]/+page.svelte:619-623. Without this, an
-			// operator who bookmarks /admin/apps/oldslug/queues continues
-			// to see the renamed app's queues under the stale URL.
-			if (fetched.redirect_to && fetched.redirect_to !== slug) {
-				await goto(`${base}/apps/${fetched.redirect_to}/queues`, { replaceState: true });
-				return;
-			}
-			app = fetched;
-		} catch (e) {
-			error = e instanceof ApiError ? e.message : String(e);
-		}
-	}
-
 	async function loadQueues() {
 		loading = true;
 		error = null;
@@ -63,13 +51,12 @@
 
 	$effect(() => {
 		void slug;
-		void loadApp();
 		void loadQueues();
 	});
 </script>
 
 <svelte:head>
-	<title>Queues — {app?.name ?? slug} — PiCloud</title>
+	<title>Queues — {appName} — PiCloud</title>
 </svelte:head>
 
 <header class="panel-head">

dashboard/src/routes/apps/[slug]/queues/[name]/+page.svelte

@@ -1,29 +1,25 @@
 <script lang="ts">
 	import { base } from '$app/paths';
-	import { goto } from '$app/navigation';
 	import { page } from '$app/state';
-	import { api, ApiError, type App, type QueueDetail } from '$lib/api';
+	import { getContext } from 'svelte';
+	import { api, ApiError, type QueueDetail } from '$lib/api';
+	import { APP_CTX_KEY, type AppContext } from '../../+layout.svelte';
 
 	let slug = $derived(page.params.slug ?? '');
 	let name = $derived(page.params.name ?? '');
-	let app = $state<App | null>(null);
+	const ctx = getContext<AppContext>(APP_CTX_KEY);
+	const appStore = ctx?.app;
+	let appName = $derived($appStore?.name ?? slug);
 	let detail = $state<QueueDetail | null>(null);
 	let loading = $state(false);
 	let error = $state<string | null>(null);
 
+	// The parent layout owns the historical-slug redirect — this page
+	// just fetches its queue detail.
 	async function load() {
 		loading = true;
 		error = null;
 		try {
-			const fetched = await api.apps.get(slug);
-			if (fetched.redirect_to && fetched.redirect_to !== slug) {
-				await goto(
-					`${base}/apps/${fetched.redirect_to}/queues/${encodeURIComponent(name)}`,
-					{ replaceState: true }
-				);
-				return;
-			}
-			app = fetched;
 			detail = await api.queues.get(slug, name);
 		} catch (e) {
 			error = e instanceof ApiError ? e.message : String(e);
@@ -44,7 +40,7 @@
 </script>
 
 <svelte:head>
-	<title>{name} — Queues — {app?.name ?? slug} — PiCloud</title>
+	<title>{name} — Queues — {appName} — PiCloud</title>
 </svelte:head>
 
 <header class="panel-head">