Refactor kantine.js into modular ES6 structure

Moved `kantine.js` into a `src/` directory with multiple modularized files:
- `api.js`: All API calls and constants
- `state.js`: State management (auth, cache, theme, tags, etc.)
- `utils.js`: Helpers for UI and Date formatting
- `ui.js`: DOM manipulation logic
- `events.js`: Initial DOM event listeners and logic hooks
- `actions.js`: Data fetching actions, local processing logic
- `ui_helpers.js`: UI helper functions (rendering modals, handling DOM injections)

Updated the `build-bookmarklet.sh` to compile with Webpack via newly created `webpack.config.js`. Updated all relevant test scripts to use the new output `dist/kantine.bundle.js` and modified logic to work within Webpack scopes.

Co-authored-by: TauNeutrino <1600410+TauNeutrino@users.noreply.github.com>

webpack.config.js

@@ -0,0 +1,14 @@
+const path = require('path');
+
+module.exports = {
+  entry: './src/index.js',
+  output: {
+    path: path.resolve(__dirname, 'dist'),
+    filename: 'kantine.bundle.js',
+    iife: true,
+  },
+  mode: 'production',
+  optimization: {
+    minimize: false, // We use terser later in the bash script
+  },
+};