Improve README example code accuracy and completeness

[Copilot]

Several README examples were incomplete, inaccurate, or referenced APIs removed in v12.0, causing confusion for new users.

Changes

• Alert Example: Added missing void main() with WidgetsFlutterBinding.ensureInitialized(); fixed const usage throughout
• Removed upgraderDevice parameter: Dropped from Upgrader constructor in v12.0 but still listed in docs
• Fixed UpgraderStoreController defaults (swapped in docs vs. actual code):
  • onFuchsia: UpgraderAppStore() → null
  • oniOS: null → UpgraderAppStore()
• Fixed Appcast Example: osVersion was referenced but never defined; replaced with Version(0, 0, 0) placeholder and added comment pointing to example/lib/main_appcast.dart for the full device_info_plus-based implementation; added oniOS alongside onAndroid
• UpgraderStoreController iOS Appcast snippet: Replaced undefined osVersion with Version(0, 0, 0)
• Minor: Added dart language tag to bare code fences; fixed missing const in Go Router snippet

static final upgrader = Upgrader(
  storeController: UpgraderStoreController(
    onAndroid: () => UpgraderAppcastStore(appcastURL: appcastURL, osVersion: Version(0, 0, 0)),
    oniOS: () => UpgraderAppcastStore(appcastURL: appcastURL, osVersion: Version(0, 0, 0)),
  ),
);

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[Copilot]

Pull request overview

This PR improves the README documentation by fixing several inaccuracies and making code examples more complete and runnable. The changes address issues introduced in v12.0 and correct long-standing documentation errors.

Changes:

• Fixed README code examples to be complete and accurate, including proper initialization patterns
• Corrected UpgraderStoreController default value documentation (swapped Fuchsia/iOS defaults)
• Removed deprecated upgraderDevice parameter documentation (removed in v12.0)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.08%. Comparing base (e8bf433) to head (f83ca46).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #538   +/-   ##
=======================================
  Coverage   93.08%   93.08%           
=======================================
  Files          12       12           
  Lines        1287     1287           
=======================================
  Hits         1198     1198           
  Misses         89       89           

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