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]/+layout.svelte

@@ -57,11 +57,24 @@
 		if (pathname.endsWith(`/apps/${slug}`) || pathname.endsWith(`/apps/${slug}/`)) {
 			return qpTab ?? 'scripts';
 		}
-		if (pathname.includes(`/apps/${slug}/users`)) return 'users';
-		if (pathname.includes(`/apps/${slug}/files`)) return 'files';
-		if (pathname.includes(`/apps/${slug}/queues`)) return 'queues';
-		if (pathname.includes(`/apps/${slug}/dead-letters`)) return 'dead-letters';
-		return 'scripts';
+		// Anchored matches: only the named subtab *exactly* (followed by
+		// `/` or end-of-string), so a hypothetical `/apps/<slug>/queues-archived`
+		// route doesn't activate the `queues` tab. Audit hardening note.
+		const root = `/apps/${slug}/`;
+		const after = pathname.startsWith(root) ? pathname.slice(root.length) : '';
+		const head = after.split('/', 1)[0];
+		switch (head) {
+			case 'users':
+				return 'users';
+			case 'files':
+				return 'files';
+			case 'queues':
+				return 'queues';
+			case 'dead-letters':
+				return 'dead-letters';
+			default:
+				return 'scripts';
+		}
 	}
 
 	async function reloadApp() {