Resume the last garden on this device (#97)

`/` always dumped you on the gardens list; a phone user who lives in one
garden had to open the list and tap in every time. Now the device
remembers the garden it was last in and `/` resumes there.

- lib/lastGarden.ts: per-device localStorage (pansy:last-garden), same
  swallow-failures rationale as the seed tray / recents. getLastGardenId
  guards against a non-positive/garbage stored value.
- The `/` route redirects to the stored garden when present, else /gardens.
- The editor records the garden on successful load, and — if it 404s
  (deleted or access revoked) — forgets it (only if it's the stored one,
  so a bad direct link can't wipe a good resume target) and bounces to the
  list, so a stale id can't trap the user on an error screen. Transient
  errors still show the retryable message.
- ApiError.isNotFound getter (mirrors isConflict/isUnauthorized).

Verified live at 390px: resume into the last garden; a stale id bounces to
/gardens and clears itself. tsc + vitest (incl. new lastGarden test) green.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

web/src/pages/GardenEditorPage.tsx

@@ -36,6 +36,8 @@ import { useJournalCounts } from '@/lib/journal'
 import { attributableLots, lotsByPlant, useSeedLots, type SeedLot } from '@/lib/seedLots'
 import { useSeedTray } from '@/lib/seedTray'
 import { usePageTitle } from '@/lib/usePageTitle'
+import { rememberLastGarden, forgetLastGarden } from '@/lib/lastGarden'
+import { ApiError } from '@/lib/api'
 
 const routeApi = getRouteApi('/gardens/$gardenId')
 
@@ -133,6 +135,24 @@ export function GardenEditorPage() {
     navigate({ search: (prev) => ({ ...prev, focus: focusedObjectId ?? undefined }), replace: true })
   }, [focusedObjectId, navigate])
 
+  // Remember this garden as the device's last-opened, so `/` resumes here next
+  // visit. Keyed on the live query: which garden EXISTS doesn't depend on the
+  // season being viewed.
+  useEffect(() => {
+    if (live.isSuccess) rememberLastGarden(gid)
+  }, [live.isSuccess, gid])
+
+  // A last garden that 404s (deleted, or access revoked) must not strand the user
+  // on an error screen the `/` resume keeps returning to: forget it (only if it's
+  // this one, so a bad direct link can't wipe a good resume target) and bounce to
+  // the list. Transient errors (500/network) fall through to the retryable screen.
+  useEffect(() => {
+    if (live.isError && live.error instanceof ApiError && live.error.isNotFound) {
+      forgetLastGarden(gid)
+      navigate({ to: '/gardens' })
+    }
+  }, [live.isError, live.error, gid, navigate])
+
   // If the focused object vanished — deleted, or a stale ?focus id on load — leave
   // focus mode so the canvas isn't stuck dimmed with no way out.
   useEffect(() => {
@@ -256,12 +276,20 @@ export function GardenEditorPage() {
   }, [])
 
   if (full.isPending) return <p className="p-6 text-sm text-muted">Loading garden…</p>
-  if (full.isError)
+  if (full.isError) {
+    // A not-found is handled by the effect above (forget + bounce to the list);
+    // say so rather than flashing a dead-end error during the redirect.
+    const gone = full.error instanceof ApiError && full.error.isNotFound
     return (
       <div className="p-6">
-        <Alert>Could not load this garden.</Alert>
+        <Alert>
+          {gone
+            ? 'That garden is no longer available — taking you to your gardens…'
+            : 'Could not load this garden.'}
+        </Alert>
       </div>
     )
+  }
 
   const g = full.data.garden
   const garden: EditorGarden = {