auto-save drafts in ProjectForm and GardenItemForm

Created by: jedmund

Summary

• ports the PostForm autosave pattern (proven in PRs #87–#90) to ProjectForm and GardenItemForm:
  • drafts auto-save 1500ms after the last edit; Cmd+S, window blur, and in-app navigation flush pending changes
  • StatusDropdown's combined trigger now shows the autosave label (Saved / Saving… / Unsaved / Save failed / Conflict — reload) for drafts; published/list-only/password-protected keep the existing manual-save UI
  • mid-save keystrokes stay dirty (capture submittingSnapshot before await, restore on success — same race fix as the post form)
  • 409 conflicts surface as Conflict — reload and stop autosaving; failed in-app navigations fall back to the unsaved-changes modal so we never silently drop edits
• replaces each form's bespoke dirty-tracking with a single JSON.stringify snapshot derived (matches PostForm)
• drops the post-save formStore.populateFromProject(saved) / populateFormData(saved) reset from the autosave path so it doesn't clobber mid-await keystrokes; manual publish still calls formStore.populateFromProject indirectly via project = savedProject for the next concurrency check
• skips formStore.validate() and the title is required check on silent autosaves; manual saves keep strict validation
• gates enabled() on title.trim() !== '' so autosave doesn't fire prematurely on a blank form

AlbumForm is intentionally untouched — albums don't have a draft/published status and the create flow needs photos before save makes sense.

Test plan

For each of ProjectForm and GardenItemForm:

• new draft: enter a title and a few words → after ~1.5s URL silently swaps to /admin/{type}/[id]/edit and the dropdown shows "Saved"; continued edits cycle through "Unsaved" → "Saving…" → "Saved" with no manual click
• existing draft: edit → autosaves silently; refresh, changes persisted
• Cmd+S during editing: trigger flashes "Saving…" then "Saved"
• Publish on a dirty draft: autosave flushes first, publish runs on top, dropdown switches to the published variant
• two-tab test: edit and save in tab A → tab B's next autosave returns 409 → trigger shows "Conflict — reload" and stops; reload works
• non-draft statuses (Project: list-only / password-protected / published; Garden: published) keep the existing manual save UI; no autosave fires
• navigate away from a dirty draft via sidebar: silent save, no UnsavedChangesModal; offline → modal appears
• empty title on new draft: no autosave fires until title is entered
• pnpm run check net error count unchanged or lower (origin/main: 118; this PR: 114)