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/lib/lastGarden.test.ts

@@ -0,0 +1,70 @@
+import { afterEach, beforeEach, describe, expect, it } from 'vitest'
+import { getLastGardenId, rememberLastGarden, forgetLastGarden } from './lastGarden'
+
+// The tests run in the node environment (no DOM), so stand up a minimal
+// in-memory localStorage rather than pull in jsdom for one thin module.
+function installStorage(impl?: Partial<Storage>) {
+  const store = new Map<string, string>()
+  const base: Storage = {
+    getItem: (k) => store.get(k) ?? null,
+    setItem: (k, v) => void store.set(k, String(v)),
+    removeItem: (k) => void store.delete(k),
+    clear: () => store.clear(),
+    key: (i) => [...store.keys()][i] ?? null,
+    get length() {
+      return store.size
+    },
+  }
+  ;(globalThis as { localStorage?: Storage }).localStorage = { ...base, ...impl }
+}
+
+beforeEach(() => installStorage())
+afterEach(() => {
+  delete (globalThis as { localStorage?: Storage }).localStorage
+})
+
+describe('lastGarden', () => {
+  it('round-trips a remembered garden id', () => {
+    expect(getLastGardenId()).toBeNull()
+    rememberLastGarden(42)
+    expect(getLastGardenId()).toBe(42)
+  })
+
+  it('rejects a non-positive or unparseable stored value', () => {
+    localStorage.setItem('pansy:last-garden', 'not-a-number')
+    expect(getLastGardenId()).toBeNull()
+    localStorage.setItem('pansy:last-garden', '0')
+    expect(getLastGardenId()).toBeNull()
+    localStorage.setItem('pansy:last-garden', '-3')
+    expect(getLastGardenId()).toBeNull()
+  })
+
+  it('forgets unconditionally with no argument', () => {
+    rememberLastGarden(7)
+    forgetLastGarden()
+    expect(getLastGardenId()).toBeNull()
+  })
+
+  it('forgets only when the stored id matches onlyIfEquals', () => {
+    rememberLastGarden(5)
+    // A 404 on a different (directly-linked) garden must not wipe the good resume.
+    forgetLastGarden(9)
+    expect(getLastGardenId()).toBe(5)
+    // A 404 on the stored garden itself does clear it.
+    forgetLastGarden(5)
+    expect(getLastGardenId()).toBeNull()
+  })
+
+  it('swallows storage failures instead of throwing', () => {
+    installStorage({
+      setItem: () => {
+        throw new Error('quota')
+      },
+      getItem: () => {
+        throw new Error('blocked')
+      },
+    })
+    expect(() => rememberLastGarden(1)).not.toThrow()
+    expect(getLastGardenId()).toBeNull() // getItem throwing → null, not a crash
+  })
+})