feat(v1.1.1-dead-letters): dashboard badge + list view

Design notes §4 makes the dashboard surface load-bearing — with no
default DL handler, users wouldn't know dead letters exist
otherwise.

New route: `apps/[slug]/dead-letters/+page.svelte` — list view
columns per the design notes:
- `created_at`, `source`, `op`, `script_id`, `attempt_count`,
  `first/last_attempt_at`, `last_error` (truncated; clickable)
- per-row Replay + Mark resolved buttons
- expandable row detail panel showing full payload (JSON) +
  full last_error
- unresolved-only filter (default on); refresh button

Per-app detail page (`apps/[slug]/+page.svelte`) grows a "Dead
letters" link in the tabs nav, with a red unresolved-count pill
when > 0. Loaded in parallel with the existing app loaders so it
doesn't slow the page.

Apps list (`apps/+page.svelte`) shows the same red pill next to
each app's name when its unresolved count > 0. Counts fetched in
parallel after the apps list lands; failures here are non-fatal
(just no badge).

API client wiring: `api.deadLetters.{count,list,get,replay,resolve}`
mirrors the v1.1.1 admin endpoints. `DeadLetterRow` type added to
the dashboard's API shape declarations.

dashboard's svelte-check passes (369 files, 0 errors, 0 warnings).

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

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

@@ -37,6 +37,20 @@
 	let domains = $state<AppDomain[]>([]);
 	let members = $state<AppMemberDto[]>([]);
 
+	/// v1.1.1 dead-letters surface — design notes §4 mandates the
+	/// dashboard surface this since there's no default handler.
+	let unresolvedDeadLetters = $state<number>(0);
+	async function loadDeadLetterCount(idOrSlug: string) {
+		try {
+			const r = await api.deadLetters.count(idOrSlug);
+			unresolvedDeadLetters = r.unresolved;
+		} catch {
+			// Non-fatal: the page renders fine without the badge if
+			// the count endpoint is unreachable (e.g. older server).
+			unresolvedDeadLetters = 0;
+		}
+	}
+
 	// Derive UI gates from the capabilities helper so the rules stay
 	// in lockstep with the backend's `can()`. canAdminApp also covers
 	// the Members + Settings + Domains-mutation tabs; canWriteApp
@@ -107,7 +121,11 @@
 			editName = app.name;
 			editDescription = app.description ?? '';
 			editSlug = app.slug;
-			const loaders: Promise<unknown>[] = [loadScripts(app.id), loadDomains(app.id)];
+			const loaders: Promise<unknown>[] = [
+				loadScripts(app.id),
+				loadDomains(app.id),
+				loadDeadLetterCount(app.id)
+			];
 			if (canAdmin) {
 				loaders.push(loadMembers(app.id), loadEligibleUsers());
 			}
@@ -421,6 +439,16 @@
 				class:active={activeTab === 'settings'}
 				onclick={() => (activeTab = 'settings')}>Settings</button
 			>
+			<a
+				class="tab-link"
+				href="{base}/apps/{slug}/dead-letters"
+				title="Dead letters — replay or resolve events that exhausted their retry policy"
+			>
+				Dead letters
+				{#if unresolvedDeadLetters > 0}
+					<span class="dl-badge">{unresolvedDeadLetters}</span>
+				{/if}
+			</a>
 		{/if}
 	</nav>
 
@@ -871,6 +899,32 @@
 		border-bottom-color: #38bdf8;
 	}
 
+	.tabs .tab-link {
+		display: inline-flex;
+		align-items: center;
+		gap: 0.4rem;
+		color: #94a3b8;
+		text-decoration: none;
+		padding: 0.6rem 1rem;
+		margin-left: auto;
+		border-bottom: 2px solid transparent;
+		font: inherit;
+	}
+	.tabs .tab-link:hover {
+		color: #e2e8f0;
+	}
+	.dl-badge {
+		display: inline-block;
+		min-width: 1.25rem;
+		padding: 0.1rem 0.4rem;
+		background: #ef4444;
+		color: #fff;
+		border-radius: 999px;
+		font-size: 0.75rem;
+		font-weight: 600;
+		text-align: center;
+	}
+
 	button {
 		background: #38bdf8;
 		color: #0b1220;