fitness: add offline support with session queue and shell caching

Cache fitness page shells and data routes in the service worker so
pages load offline. Queue finished workouts in IndexedDB when the
POST fails and auto-flush them on reconnect. Show an offline banner
on the completion screen so the user knows their workout will sync.

src/routes/fitness/[workout=fitnessWorkout]/[active=fitnessActive]/+page.svelte

@@ -13,6 +13,7 @@
 	import { estimateWorkoutKcal } from '$lib/data/kcalEstimate';
 	import { estimateCardioKcal } from '$lib/data/cardioKcalEstimate';
 	import ExerciseName from '$lib/components/fitness/ExerciseName.svelte';
+	import { queueSession } from '$lib/offline/fitnessQueue';
 	import SetTable from '$lib/components/fitness/SetTable.svelte';
 	import ExercisePicker from '$lib/components/fitness/ExercisePicker.svelte';
 	import SyncIndicator from '$lib/components/fitness/SyncIndicator.svelte';
@@ -35,6 +36,7 @@
 	/** @type {any[]} */
 	let templateDiffs = $state([]);
 	let templateUpdateStatus = $state('idle'); // 'idle' | 'updating' | 'done'
+	let offlineQueued = $state(false);
 
 	let useGps = $state(gps.isTracking);
 
@@ -214,9 +216,15 @@
 				const d = await res.json();
 				completionData = buildCompletion(sessionData, d.session);
 				computeTemplateDiff(completionData);
+			} else {
+				await queueSession(sessionData);
+				offlineQueued = true;
+				completionData = buildCompletion(sessionData, { _id: null });
 			}
-		} catch (err) {
-			console.error('[finish] fetch error:', err);
+		} catch {
+			await queueSession(sessionData);
+			offlineQueued = true;
+			completionData = buildCompletion(sessionData, { _id: null });
 			await sync.onWorkoutEnd();
 		}
 	}
@@ -526,6 +534,9 @@
 				</div>
 			{/if}
 			<p class="completion-name">{completionData.name}</p>
+			{#if offlineQueued}
+				<p class="offline-banner">{t('workout_saved_offline', lang)}</p>
+			{/if}
 		</div>
 
 		<div class="completion-stats">
@@ -651,8 +662,8 @@
 			</div>
 		{/if}
 
-		<button class="done-btn" onclick={() => goto(`/fitness/${sl.history}/${completionData.sessionId}`)}>
-			{t('view_workout', lang)}
+		<button class="done-btn" onclick={() => goto(offlineQueued ? `/fitness/${sl.workout}` : `/fitness/${sl.history}/${completionData.sessionId}`)}>
+			{offlineQueued ? t('done', lang) : t('view_workout', lang)}
 		</button>
 	</div>
 
@@ -794,6 +805,14 @@
 		font-size: 0.9rem;
 		color: var(--color-text-secondary);
 	}
+	.offline-banner {
+		margin: 0.5rem 0 0;
+		padding: 0.4rem 0.8rem;
+		font-size: 0.8rem;
+		color: var(--nord0);
+		background: var(--nord13);
+		border-radius: 0.4rem;
+	}
 	.pr-badge {
 		display: inline-flex;
 		align-items: center;