Fix Media Manager critical syntax errors and enforce ES5 architecture

- Resolved persistent 'SyntaxError: Unexpected token class' by refactoring 'MediaState' and 'UI' classes in MediaManager.html to standard ES5 function constructors.

- Resolved 'SyntaxError: Unexpected identifier src' by rewriting 'createCard' to use 'document.createElement' instead of template strings for dynamic media elements.

- Consolidated script tags in MediaManager.html to prevent Apps Script parser merge issues.

- Updated docs/ARCHITECTURE.md and MEMORY.md to formally document client-side constraints (No ES6 classes, strict DOM manipulation for media).

- Note: Google Drive video animate-on-hover functionality is implemented but currently pending verification/fix.

docs/ARCHITECTURE.md

@@ -141,3 +141,19 @@ We implemented a "Sidebar-First" architecture for product media to handle the co
     - Calculates checksums to avoid re-uploading duplicate images.
     - Uses Shopify's "Staged Uploads" -> "Create Media" mutation flow.
 
+### 8. Apps Script & HTML Service Constraints
+
+When working with `HtmlService` (client-side code), the environment differs significantly from the server-side V8 runtime.
+
+1.  **Server-Side (`.ts`/`.gs`)**:
+    -   **Runtime**: V8 Engine.
+    -   **Syntax**: Modern ES6+ (Classes, Arrow Functions, `const`/`let`) is fully supported.
+    -   **Recommendation**: Use standard TypeScript patterns.
+
+2.  **Client-Side (`.html` served via `createHtmlOutputFromFile`)**:
+    -   **Runtime**: Legacy Browser Environment / Strict Caja Sanitization.
+    -   **Constraint**: The parser often chokes on ES6 `class` syntax and complex template strings inside HTML attributes.
+    -   **Rule 1**: **NO ES6 CLASSES**. Use ES5 `function` constructors and `prototype` methods.
+    -   **Rule 2**: **NO Complex Template Strings in Attributes**. Do not use `src="${var}"` if the variable contains a URL. Use `document.createElement` and set properties (e.g., `element.src = value`) programmatically.
+    -   **Rule 3**: **Unified Script Tags**. Consolidate scripts into a single block where possible to avoid parser merge errors.
+    -   **Rule 4**: **Var over Let/Const**. Top-level variables should use `var` or explicit `window` assignment to ensure they are accessible to inline HTML handlers (e.g., `onclick="handler()"`).