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>

package.json

@@ -0,0 +1,6 @@
+{
+  "devDependencies": {
+    "jsdom": "^28.1.0",
+    "webpack-cli": "^6.0.1"
+  }
+}