docs: add Media Manager V2 architecture and mockup

docs/MEDIA_MANAGER_DESIGN.md

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+# Media Manager V2 Design & Architecture
+
+## Overview
+The **Media Manager V2** transforms the product image management experience from a simple upload utility to a full-featured "WYSIWYG" editor. It introduces a persistent "Draft" state, drag-and-drop reordering, and a robust generic synchronization engine that reconciles state between Google Drive (Source of Truth) and Shopify.
+
+## UI UX Design
+
+### Launch Logic
+To work around Google Apps Script limitations (triggers cannot open modals):
+1.  **Watcher Sidebar**: A lightweight sidebar remains open, polling for selection changes.
+2.  **Context Action**: When a user selects a cell in **Column A** (Product Image), the sidebar presents a large **"Edit Media"** button.
+3.  **Modal**: Clicking the button launches the full Media Manager Modal.
+
+### Interface Features
+-   **Grid Layout**: Drag-and-drop sortable grid.
+-   **Badges**:
+    -   ☁️ **Drive Only**: New uploads or files not yet synced.
+    -   🛍️ **Shopify Only**: Media found on Shopify but missing from Drive (will be backfilled).
+    -   ✅ **Synced**: Verifiable link between Drive and Shopify.
+-   **Video Support**:
+    -   Grid: Videos play silently loop (`muted autoplay`).
+    -   Preview: Full modal with controls.
+-   **Details Mode**: A togglable text view listing pending operations (e.g., "Deleting 2 files, Reordering 3...").
+
+![Media Manager Mockup](./images/media_manager_mockup.png)
+
+## Data Architecture
+
+### 1. Naming Convention
+Files in Drive function as the source of truth for order.
+-   **Pattern**: `[SKU]_[Index].[Extension]`
+-   **Example**: `TSHIRT-001_0001.jpg`, `TSHIRT-001_0002.mp4`
+-   **Padding**: 4 digits to support >10 items cleanly.
+
+### 2. Session Recovery (Draft State)
+To prevent data loss during browser refreshes or crashes, the edit state is persisted immediately to `UserProperties`.
+
+-   **Storage**: `PropertiesService.getUserProperties()`
+-   **Key**: `MEDIA_SESSION_[SKU]`
+-   **Schema**:
+    ```json
+    {
+      "schemaVersion": 1,
+      "timestamp": 1234567890,
+      "sku": "SKU-123",
+      "items": [
+        {
+          "id": "drive_file_id_or_shopify_id",
+          "source": "drive|shopify|new",
+          "filename": "original_name.jpg",
+          "status": "active|deleted|staged",
+          "thumbnail": "data:image..."
+        }
+      ]
+    }
+    ```
+
+### 3. Synchronization Logic (Two-Way Reconcile)
+
+#### Phase A: Load & Match (Read-Only)
+Executed when opening the manager.
+1.  **Fetch**: Get all Drive Files in SKU folder and all Shopify Media via GraphQL.
+2.  **Match**:
+    -   **Strong Verification**: `Drive.appProperties.shopify_media_id === Shopify.media.id`
+    -   **Legacy Fallback**: `Drive.name === Shopify.filename` (Only if no ID match)
+3.  **Conflict Resolution**: If duplicates found, prefer high-res/latest.
+
+#### Phase B: Save (Transactional)
+Executed when user clicks "Save".
+1.  **Delete**: Process items marked `deleted` (Remove from Shopify & Trash in Drive).
+2.  **Backfill**: Download "Shopify Only" items to Drive -> Set `appProperties`.
+3.  **Upload**: Upload "Drive Only" items -> Create Media -> Set `appProperties`.
+4.  **Reorder**: Execute `productReorderMedia` GraphQL mutation with final ID list.
+5.  **Finalize**:
+    -   Rename all Drive files to `SKU_{index}` sequence.
+    -   Clear `MEDIA_SESSION_[SKU]` property.
+
+## Technical Components
+
+### Frontend
+-   **HTML/CSS**: Glassmorphism aesthetic (Inter font, backdrop-filter).
+-   **JS**: Vanilla JS with HTML5 Drag & Drop API.
+
+### Backend Services
+-   **`MediaService`**: Orchestrates the Phase A/B logic.
+-   **`ShopifyMediaService`**: Handles GraphQL mutations (`productCreateMedia`, `productReorderMedia`).
+-   **`GASDriveService`**: Manages File renaming and `appProperties` metadata.
+
+## Future Proofing
+-   **Metadata**: We avoid relying on file hashes/sizes due to Shopify's aggressive image compression. We rely strictly on stored IDs (`appProperties`) where possible.
+-   **Scale**: Pagination may be needed if SKUs usually exceed 50 images (current limit 250 in GraphQL).
+
+## Development Roadmap
+- [ ] **Backend Implementation**
+    - [ ] Update `getMediaForSku` to return combined state (Drive + Shopify + Session)
+    - [ ] Implement `saveMediaChanges(sku, changes)` transaction logic
+        - [ ] Renaming files (`SKU_####.ext`)
+        - [ ] Deleting/Trashing files
+        - [ ] Uploading/Backfilling
+    - [ ] Implement Session Recovery (Read/Write `UserProperties`)
+- [ ] **Frontend Implementation**
+    - [ ] **Watcher Sidebar**: Create `MediaSidebar.html` to poll for selection.
+    - [ ] **Manager Modal**: Refactor `MediaManager.html`.
+        - [ ] State Management (Staging)
+        - [ ] Drag-and-Drop Grid
+        - [ ] Preview Modal (Image + Video)
+        - [ ] "Details..." View
+- [ ] **Verification**
+    - [ ] Manual Test: Drag & Drop ordering
+    - [ ] Manual Test: Save & Sync
+    - [ ] Manual Test: Session Recovery (Reload browser mid-edit)