Add configurable min-width handling for ggsql widgets

[cpsievert]

Summary

This changes the ggsql htmlwidget so narrow-width scaling is an explicit widget option instead of a hard-coded frontend rule.

• adds min_width to ggsql_widget() and makes ggsql_widget(writer, spec, ...) the public widget construction API
• updates the widget binding to read min_width from the htmlwidget payload and apply scaled mode only when that option is present
• keeps the frontend behavior intentionally simple by toggling a ggsql-scaled host class instead of publishing detailed sizing metadata
• adjusts the scaled transform origin to left center

To try the behavior interactively in Shiny, run an app like this and drag the right pane narrower to compare default behavior vs min_width = 450:

library(ggsql)
library(shiny)

reader <- duckdb_reader()
ggsql_register(reader, mtcars, "cars")

spec <- ggsql_execute(
  reader,
  "SELECT * FROM cars VISUALISE mpg AS x DRAW histogram"
)

ui <- fluidPage(
  tags$head(
    tags$style(HTML(
      "
      .demo-grid {
        display: grid;
        gap: 1rem;
        grid-template-columns: repeat(auto-fit, minmax(320px, 1fr));
      }
      .demo-card {
        border: 1px solid #d9d9d9;
        border-radius: 10px;
        padding: 0.75rem;
        background: white;
      }
      .demo-pane {
        width: 280px;
        min-width: 180px;
        max-width: 100%;
        height: 360px;
        resize: horizontal;
        overflow: auto;
        border: 1px dashed #b8b8b8;
        padding: 0.5rem;
        background: #fafafa;
      }
      .demo-pane ggsql-vega {
        background: white;
      }
      "
    ))
  ),
  div(
    class = "demo-grid",
    div(
      class = "demo-card",
      h3("Default widget sizing"),
      div(class = "demo-pane", ggsqlOutput("plain", width = "100%", height = "100%"))
    ),
    div(
      class = "demo-card",
      h3("Widget with min_width = 450"),
      div(class = "demo-pane", ggsqlOutput("scaled", width = "100%", height = "100%"))
    )
  )
)

server <- function(input, output, session) {
  output$plain <- renderGgsql({ spec })
  output$scaled <- renderGgsql({
    ggsql_widget(vegalite_writer(), spec, min_width = 450)
  })
}

shinyApp(ui, server)