Fix localStorage autosave silent failure — warn user when storage is full

[zntznt]

Problem

js/app.js:486,507 — both _commitReplace and _commit silently swallow localStorage errors:

try { localStorage.setItem('sim_autosave', this._lastState); } catch {}

If storage is full (5-10MB quota), autosave fails silently. User closes the browser and loses work with no warning.

Fix

Replace both sites with a one-shot toast warning:

try {
  localStorage.setItem('sim_autosave', this._lastState);
  this._autosaveWarned = false;
} catch {
  if (!this._autosaveWarned) {
    this._toast('Autosave failed. Storage may be full. Save your work via File, Export, then clear some browser data.');
    this._autosaveWarned = true;
  }
}

Initialize _autosaveWarned = false in the constructor (near existing boolean flags at line 25).

The flag ensures one toast per failure streak — not one per _commit call. On a successful save, the flag resets so future failures re-arm the warning.

What's NOT changed

• Library/component saves (app-library.js:101,112) already handle errors correctly with catch { return false } + caller toast.
• Other localStorage.setItem calls for non-critical flags (tour seen, welcome seen, MC hint, palette sections, timeline compare) — these are UI state flags where silent failure is acceptable.

Effort

~10 minutes. 1 file (js/app.js). ~12 lines changed.

[zntznt]

Implementation Order

This is Phase 1 · Step 1.4 — implement after #43.

Priority: MODERATE
Phase: 1 (Safety + Data Integrity)
Order: 4 of 10
Depends on: Nothing (independent)
Effort: ~10 minutes