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>

build-bookmarklet.sh

@@ -6,7 +6,7 @@ set -e
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 DIST_DIR="$SCRIPT_DIR/dist"
 CSS_FILE="$SCRIPT_DIR/style.css"
-JS_FILE="$SCRIPT_DIR/kantine.js"
+JS_FILE="$SCRIPT_DIR/dist/kantine.bundle.js"
 FAVICON_FILE="$SCRIPT_DIR/favicon.png"
 
 # === VERSION ===