fix(pkg-r): better support for modules.

[cpsievert]

Closes #169

This PR introduces proper namespace handling for QueryChat within Shiny (for R) modules:

Changes to $ui() and $sidebar() methods:

1. Added ns parameter: Users can now explicitly pass a Shiny namespacing function (shiny::NS()) when calling these methods within a module UI function
2. Automatic namespace detection: When called in a module server context (e.g., with renderUI()), the namespace is automatically inferred from the session
3. Error handling: Added validation to prevent users from incorrectly passing id directly, with helpful error messages guiding them to use ns instead

Implementation details:

• New namespaced_id() helper function that checks for a reactive domain and applies namespacing when available
• Updated documentation for the id field to clarify it's an instance ID rather than specifically a module ID
• Proper parameter documentation for the new ns argument

Usage

When using QueryChat in a module UI function, pass the namespacing function:

test_module_ui <- function(id) {
  ns <- NS(id)
  qc$ui(ns = ns)
}

When using renderUI() in a module server, the namespacing is automatic:

output$querychat_sidebar <- renderUI({
  qc$ui()  # Namespace inferred automatically
})