chore(riverpod): establish migration base infrastructure

[Sembauke]

Summary

• Establishes the Riverpod migration base branch.
• Replaces Stacked app bootstrap wiring with local compatibility primitives:
  • App router (onGenerateRoute)
  • Navigation/snackbar/dialog wrappers
  • GetIt locator registration without Stacked runtime
• Adds the migration coordination document and provider bridge used by follow-up feature branches.

Scope (base only)

• Foundation/bootstrap changes only.
• No feature-domain migration content is included here.

Follow-up PR stack

• This PR is the parent/base for the per-feature migration PRs:
  • Learn
  • Podcast
  • News
  • Code Radio
  • Login
  • Profile
  • Settings
  • Shared UI

[Sembauke]

Closing this base PR per request. Keeping the base branch for stacked feature PR targeting.