4.0.1 — README, screenshots and AI assistant guide

Fixed

• showMapLocationPicker threw when an existing onNext popped with a
  result. Code carried over from 3.x as onNext: (r) => Navigator.pop(context, r)
  popped the route with a GeocodingResult, but the route was typed
  PickedPlace, so it failed with an assertion in debug and a TypeError in
  release — on exactly the migration path the migration guide recommends. Any
  popped value is now accepted, and the future still resolves to the
  PickedPlace the user confirmed.
• A geocoder given to MapLocationPickerController was silently discarded.
  MapLocationPicker(controller: c) without its own geoCodingConfig replaced
  the one configured on c with the default, so a proxy, custom HTTP client or
  test fake stopped being used as soon as the picker mounted. A geocoder passed
  to the widget still takes precedence.
• The back button and the search field now share one row, so they are centred
  on the same line; they previously sat at different heights.
• bottomCardTitle now starts at the same left edge as the address below it.
• The default search hint is now "Search for a place or address". The previous
  text was clipped mid-sentence whenever the back button was shown.

Documentation

• README rewritten: a step-by-step getting-started guide, a section for each
  feature, a configuration reference, and troubleshooting. Every factual
  claim was then audited against the source, which corrected several errors
  carried over from the old README:
  • The iOS AppDelegate snippet didn't match current Flutter templates.
  • Android was listed as API 21, when google_maps_flutter needs 24.
  • The iOS 14 deployment target that apps created with Flutter 3.38 must set
    was never mentioned.
  • The web Maps script was loaded with async/loading=async;
    google_maps_flutter_web needs it loaded synchronously. The example app
    is fixed too.
  • The key-restriction sample used dart:io, which throws on web, and the
    guidance didn't mention that the Geocoding API rejects
    referrer-restricted keys.
  • How key problems map to error kinds is now documented per API. With
    Places API (New) (search, details, nearby), a wrong key arrives as
    invalidRequest (HTTP 400, "API key not valid"). The Geocoding API
    reports it as requestDenied, and an empty key fails search with
    unknown in debug builds.
• MIGRATION_GUIDE.md: the deprecation note now says all four deprecated
  options, not "both", are removed in 5.0.0.
• New screenshots rendered from the real widgets, with a reproducible
  generator in tool/screenshots. The pub.dev gallery grows from three
  images to eight, and the package archive shrinks from about 10 MB to under
  1 MB.
• New "Using with AI coding assistants" section: project rules for
  CLAUDE.md, AGENTS.md, Copilot, Cursor and Gemini, plus seven copy-paste
  prompts for integrating, embedding, migrating, localizing, debugging and
  testing. Each prompt was checked by having an assistant follow it without
  access to the package source, then compiling the result.
• Added llms.txt, a compact and authoritative 4.x API reference for AI
  assistants.
• The package description on pub.dev now says what the package does.