fix: cache preview PNG bytes to avoid regeneration on apply

[Copilot]

Clicking "Apply" after previewing regenerated the image from scratch — wasting Gemini API quota and risking a different result than what was previewed (non-deterministic generation + sun position drift between calls).

Changes

AppState — new cache field

• Added cached_preview: Mutex<Option<Vec<u8>>> to hold the last previewed PNG bytes in memory

commands/mod.rs — populate & consume cache

• preview_image_with_config / preview_image_enhanced_with_config: now accept State<'_, AppState> and store generated bytes into the cache after encoding (no extra clone)
• apply_to_lockscreen_with_config: takes cache via take() (consuming it atomically); if present, writes directly to disk and applies — skipping all regeneration; falls back to run_once_with_config if cache is empty

scheduler/mod.rs — cache invalidation

• run_once_with_config clears the cache at the start of every scheduled/manual update cycle, preventing stale preview bytes from being applied by the scheduler
• output_image_path promoted to pub so the apply command can resolve the output path without duplicating logic

Flow

preview_image_with_config(cfg, state)
  → render → encode → state.cached_preview = Some(bytes)   // cache populated

apply_to_lockscreen_with_config(cfg, app, state)
  → state.cached_preview.take()
      Some(bytes) → fs::write + set_lockscreen_image        // no regeneration
      None        → run_once_with_config(app, cfg)          // fallback