fix(backup): pause revoked backups and tell users to reconnect

Google answers a revoked or expired refresh token with invalid_grant.
Every save then retried the dead token, and reconnecting never cleared
the old error, so it kept showing on Backup Settings afterwards.

- The Google Drive and Dropbox OAuth callbacks clear lastError when a
  provider is reconnected.
- An invalid_grant, or a missing refresh token, now pauses the
  integration with a readable "reconnect" message instead of retrying
  it on every catch update. Other failures still retry as before.
- A banner on every page and an alert on the Pokédex page point to
  Backup Settings, which shows a "Reconnect needed" badge. The Pokédex
  page re-checks backup status after each export, because saving a
  catch record also exports on the server and may pause a provider
  first.
- Offline sync status and the "Save all artwork" link move from every
  page to a new /offline-guide page, linked from the user menu and the
  home and welcome pages. Only the offline read-only banner stays
  sitewide.
- Unit tests cover every export path and the backup status store. BDD
  covers revocation and reconnecting for both providers, and the
  offline guide. The mock provider can now reject token refreshes, and
  mock control calls fail loudly if a stale mock is reused. Coverage
  thresholds are raised to the new baseline.

src/routes/pokedex/[id]/+page.svelte

@@ -17,6 +17,14 @@
 	import type { Pokedex } from '$lib/models/Pokedex';
 	import type { PageData } from './$types';
 	import { requestOfflineSync } from '$lib/stores/offlineSync';
+	import { get } from 'svelte/store';
+	import {
+		PROVIDER_LABELS,
+		backupsNeedingReconnect,
+		markReconnectNeeded,
+		refreshBackupStatus
+	} from '$lib/stores/backupStatus';
+	import type { ExportProvider } from '$lib/models/PokedexExportIntegration';
 	import type { SharedCombinedData } from '$lib/models/SharedPokedex';
 
 	export let data: PageData;
@@ -66,6 +74,8 @@
 		lastSuccessfulFlushAt: null
 	};
 	let lastOfflineSyncFlush: number | null = null;
+	// Backup providers that just refused this page's export because their access was revoked.
+	let reconnectToastLabels: string[] = [];
 	let exportAfterFlush = false;
 	let exportInFlight = false;
 	let exportTimer: ReturnType<typeof setTimeout> | null = null;
@@ -110,6 +120,24 @@
 					console.error('Auto-export failed:', response.status, body);
 					return;
 				}
+				const result = (await response.json().catch(() => null)) as {
+					failed?: Array<{ provider: ExportProvider; reconnectRequired?: boolean }>;
+				} | null;
+				const revoked = (result?.failed ?? []).filter((failure) => failure.reconnectRequired);
+				const alreadyPaused = new Set(get(backupsNeedingReconnect));
+				if (revoked.length > 0) {
+					markReconnectNeeded(revoked.map((failure) => failure.provider));
+				} else {
+					// Saving a catch record also exports on the server, and that export may already have
+					// paused a provider, leaving this export nothing to report. Re-read the status to catch it.
+					await refreshBackupStatus();
+				}
+				const newlyPaused = get(backupsNeedingReconnect).filter(
+					(provider) => !alreadyPaused.has(provider)
+				);
+				if (newlyPaused.length > 0) {
+					reconnectToastLabels = newlyPaused.map((provider) => PROVIDER_LABELS[provider]);
+				}
 				if (exportGeneration === exportInFlightGeneration) {
 					exportAfterFlush = false;
 				}
@@ -514,6 +542,24 @@
 	});
 </script>
 
+{#if reconnectToastLabels.length > 0}
+	<!-- Above DaisyUI's modal (z-index 999) so the alert stays usable over an open Pokémon dialog. -->
+	<div class="toast toast-end z-[1000]">
+		<div class="alert alert-warning" role="alert" data-testid="backup-reconnect-toast">
+			<span>
+				Backups to {reconnectToastLabels.join(' and ')} have stopped because access expired or was revoked.
+			</span>
+			<a class="btn btn-sm" href="/backup-settings">Reconnect</a>
+			<button
+				type="button"
+				class="btn btn-sm btn-ghost"
+				aria-label="Dismiss"
+				on:click={() => (reconnectToastLabels = [])}>✕</button
+			>
+		</div>
+	</div>
+{/if}
+
 <svelte:head>
 	<title>{pokedex ? `${pokedex.name} - Living Dex Tracker` : 'Pokédex - Living Dex Tracker'}</title>
 </svelte:head>