docs(getting-started): remove dangerous 'flutter create' advice and document run_linux.sh #1501

Problem

In docs/getting-started.md:

1. Destructive command advice: Lines 34-39 suggest running:

   flutter create . --project-name querya_desktop --platforms=linux,windows,macos

   if platform folders are missing. All platform folders (linux/, windows/, macos/) in this repository contain custom native C++ code (bitsdojo_window, custom refresh rate headers, runner modifications, icons, entitlements). Running flutter create . risks overwriting and corrupting these native sources with boilerplate Flutter templates.
2. Missing recommendation of run_linux.sh:
   docs/getting-started.md lists flutter run -d linux and GDK_BACKEND=x11 flutter run -d linux, but does not highlight ./run_linux.sh as the recommended launcher script for Linux developers (it automatically verifies pkg-config --exists libsecret-1 and sets GDK_BACKEND=x11).
3. Flutter version clarification:
   Clarify that CI pins Flutter 3.47.1 (stable) and Dart 3.13.x to prevent "works on my machine" differences.

Scope

• In docs/getting-started.md:
  • Remove the flutter create . snippet completely.
  • Emphasize running ./run_linux.sh on Linux.
  • Explicitly document required Linux build dependencies (clang cmake ninja-build pkg-config libgtk-3-dev libsecret-1-dev).

Acceptance

• No suggestion to run flutter create . on existing codebase.
• Linux dev instructions clearly steer users towards ./run_linux.sh.