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/state.js

@@ -0,0 +1,25 @@
+import { getISOWeek } from './utils.js';
+
+export let allWeeks = [];
+export let currentWeekNumber = getISOWeek(new Date());
+export let currentYear = new Date().getFullYear();
+export let displayMode = 'this-week';
+export let authToken = localStorage.getItem('kantine_authToken');
+export let currentUser = localStorage.getItem('kantine_currentUser');
+export let orderMap = new Map();
+export let userFlags = new Set(JSON.parse(localStorage.getItem('kantine_flags') || '[]'));
+export let pollIntervalId = null;
+export let langMode = localStorage.getItem('kantine_lang') || 'de';
+export let highlightTags = JSON.parse(localStorage.getItem('kantine_highlightTags') || '[]');
+
+export function setAllWeeks(weeks) { allWeeks = weeks; }
+export function setCurrentWeekNumber(week) { currentWeekNumber = week; }
+export function setCurrentYear(year) { currentYear = year; }
+export function setDisplayMode(mode) { displayMode = mode; }
+export function setAuthToken(token) { authToken = token; }
+export function setCurrentUser(user) { currentUser = user; }
+export function setOrderMap(map) { orderMap = map; }
+export function setUserFlags(flags) { userFlags = flags; }
+export function setPollIntervalId(id) { pollIntervalId = id; }
+export function setLangMode(lang) { langMode = lang; }
+export function setHighlightTags(tags) { highlightTags = tags; }