fix: dispose auth subscription and TextEditingControllers (closes #173, #174)

[EstebanCastel]

Context

This PR is part of an academic analysis of Flutter Catalog conducted for a
mobile development course (ISIS3510, Universidad de los Andes). The focus is
correctness rather than new features. Both changes are confined to the two
files that the linked issues describe; no demo behavior or educational
content is altered beyond what the issues already discussed.

Summary

Closes #173 and #174. Two independent memory-leak fixes:

1. lib/routes/firebase_login_ex.dart (#173)

_auth.authStateChanges().listen(...) previously returned a StreamSubscription
that was discarded, and the listener mutated this._user without setState,
so:

• the subscription leaked every time the route was opened, and
• the UI never reflected auth state changes pushed through the stream
  (sign-out from another tab, token expiration, etc.).

The fix:

• Stores the subscription in a _authSub field.
• Overrides dispose() to cancel it.
• Calls setState from the listener so the displayed status text reacts to
  external auth state changes.
• Adds a mounted guard before setState so callbacks that race with
  navigation do not touch a disposed widget.

2. lib/routes/networking_rest_api_send_ex.dart (#174)

Three TextEditingController instances were created in initState() but
the widget never overrode dispose(). TextEditingController is a
ChangeNotifier retained by every TextField that listens to it. The fix
overrides dispose() and releases the three controllers.

Test plan

• git diff reviewed — only the two files are touched, additions only
  (no behavior removed from the demos).
• flutter analyze lib/routes/firebase_login_ex.dart lib/routes/networking_rest_api_send_ex.dart
  — could not run locally because the project requires Flutter SDK >=3.11
  and the local environment is on 3.10. The diff is small and uses only
  standard StatefulWidget patterns (no Dart 3.11 syntax).
• Manual run of the Firebase Login demo on a physical device to confirm
  that the status text now updates immediately when auth state changes
  through the stream.
• DevTools Memory snapshot to confirm _FirebaseLoginExampleState and
  _RestApiSendExampleState instances are GC'd after navigating away from
  each demo.

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