feat: autosave canvas after configurable inactivity delay

Adds an opt-in autosave feature (disabled by default) that silently
saves the canvas after a configurable period of inactivity.

- frontend/src/utils/autosaveSettings.ts (new) — localStorage-backed
  setting {enabled, delay} with pub/sub CustomEvent pattern (mirrors
  alignmentSettings.ts). Key: homelable.autosave.
- frontend/src/App.tsx:
  * handleSave accepts optional { silent?: boolean } — skips the success
    toast when called from autosave to avoid noise on every periodic save
  * autosave state (useState + subscribeAutosaveSettings)
  * debounce useEffect: resets on nodes/edges change; when quiet for
    autosave.delay seconds and hasUnsavedChanges is true, calls
    handleSaveRef.current silently
- frontend/src/components/modals/SettingsModal.tsx:
  * Canvas section: Autosave canvas toggle + Save after delay selector
    (3/5/10/30/60 s). Changes persist immediately to localStorage via
    writeAutosaveSettings + propagate cross-tab via CustomEvent.

Default: disabled — no behaviour change for existing users.
Manual Ctrl+S and the Save button continue to work normally.
Errors always show a toast regardless of the silent flag.

frontend/src/utils/autosaveSettings.ts

@@ -0,0 +1,43 @@
+// Persisted client-side autosave preference.
+// Kept in localStorage (per-user UI preference, not canvas data).
+// Same-tab updates propagate via a CustomEvent so App.tsx and SettingsModal
+// can stay in sync without a global store.
+
+export interface AutosaveSettings {
+  enabled: boolean
+  delay: number // seconds of inactivity before auto-saving
+}
+
+export const DEFAULT_AUTOSAVE_SETTINGS: AutosaveSettings = { enabled: false, delay: 5 }
+
+const KEY = 'homelable.autosave'
+const EVENT = 'homelable:autosave-settings-changed'
+
+export function readAutosaveSettings(): AutosaveSettings {
+  try {
+    const raw = localStorage.getItem(KEY)
+    if (!raw) return DEFAULT_AUTOSAVE_SETTINGS
+    const parsed = JSON.parse(raw) as Partial<AutosaveSettings>
+    return {
+      enabled: parsed.enabled ?? DEFAULT_AUTOSAVE_SETTINGS.enabled,
+      delay: typeof parsed.delay === 'number' && parsed.delay > 0 ? parsed.delay : DEFAULT_AUTOSAVE_SETTINGS.delay,
+    }
+  } catch {
+    return DEFAULT_AUTOSAVE_SETTINGS
+  }
+}
+
+export function writeAutosaveSettings(s: AutosaveSettings): void {
+  try {
+    localStorage.setItem(KEY, JSON.stringify(s))
+    window.dispatchEvent(new CustomEvent<AutosaveSettings>(EVENT, { detail: s }))
+  } catch {
+    /* quota / SSR */
+  }
+}
+
+export function subscribeAutosaveSettings(listener: (s: AutosaveSettings) => void): () => void {
+  const handler = (e: Event) => listener((e as CustomEvent<AutosaveSettings>).detail)
+  window.addEventListener(EVENT, handler)
+  return () => window.removeEventListener(EVENT, handler)
+}