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>

src/api.js

@@ -0,0 +1,14 @@
+import { API_BASE, GUEST_TOKEN, CLIENT_VERSION } from './constants.js';
+
+export function apiHeaders(token) {
+    return {
+        'Authorization': `Token ${token || GUEST_TOKEN}`,
+        'Accept': 'application/json',
+        'Content-Type': 'application/json',
+        'X-Client-Version': CLIENT_VERSION
+    };
+}
+
+export function githubHeaders() {
+    return { 'Accept': 'application/vnd.github.v3+json' };
+}