[newsfeed] refactor: migrate to centralized HTTPFetcher (#4023)

This migrates the Newsfeed module to use the centralized HTTPFetcher
class (introduced in #4016), following the same pattern as the Calendar
module.

This continues the refactoring effort to centralize HTTP error handling
across all modules.

## Changes

**NewsfeedFetcher:**
- Refactored from function constructor to ES6 class (like the calendar
module in #3959)
- Replaced manual fetch() + timer handling with HTTPFetcher composition
- Uses structured error objects with translation keys
- Inherits smart retry strategies (401/403, 429, 5xx backoff)
- Inherits timeout handling (30s) and AbortController

**node_helper.js:**
- Updated error handler to use `errorInfo.translationKey`
- Simplified property access (`fetcher.url`, `fetcher.items`)

**Cleanup:**
- Removed `js/module_functions.js` (`scheduleTimer` no longer needed)
- Removed `#module_functions` import from package.json

## Related

Part of the HTTPFetcher migration effort started in #4016.
Next candidate: Weather module (client-side → server-side migration).

js/module_functions.js

@@ -1,18 +0,0 @@
-/**
- * Schedule the timer for the next update
- * @param {object} timer The timer of the module
- * @param {bigint} intervalMS interval in milliseconds
- * @param {Promise} callback function to call when the timer expires
- */
-const scheduleTimer = function (timer, intervalMS, callback) {
-	if (process.env.mmTestMode !== "true") {
-		// only set timer when not running in test mode
-		let tmr = timer;
-		clearTimeout(tmr);
-		tmr = setTimeout(function () {
-			callback();
-		}, intervalMS);
-	}
-};
-
-module.exports = { scheduleTimer };