feat(offcanvas): show_offcanvas() accepts a character id

[gadenbuie]

Closes #1345

Summary

show_offcanvas() previously only accepted a bslib_offcanvas object. Any other input, including a bare string, was silently coerced into body content for a new anonymous panel, unlike its id-oriented siblings hide_offcanvas()/toggle_offcanvas(). This made show_offcanvas("panel") do the wrong thing instead of revealing the existing panel.

show_offcanvas() now dispatches on the type of its first argument:

• character(1) -> the id of a panel already in the UI (reveals it)
• list-like/tag content (e.g. htmltools::HTML(), tagList()) -> wrapped into a new anonymous panel (today's old behavior, minus bare characters)
• a bslib_offcanvas object -> rendered + shown (unchanged)

Since a plain character string containing whitespace or being empty cannot be a valid HTML id, and is almost certainly mistaken body text, show_offcanvas() now errors with actionable guidance in that case rather than silently misinterpreting it. htmltools::HTML() is explicitly excluded from the id path (it is a character vector under the hood) so it is still treated as body content.

No client-side (srcts/) changes were needed: showing an existing panel by id sends the same {method: "toggle", value: "show"} message toggle_offcanvas(id, show = TRUE) already sends.

Reproducible Shiny app

library(shiny)
library(bslib)

ui <- page_fluid(
  actionButton("show_existing", "Show existing panel"),
  actionButton("show_html", "Show HTML body"),
  actionButton("show_object", "Show new offcanvas object"),
  offcanvas(
    "This panel is declared in the UI.",
    id = "existing-panel",
    title = "Existing panel"
  )
)

server <- function(input, output, session) {
  observeEvent(input$show_existing, {
    show_offcanvas("existing-panel")
  })

  observeEvent(input$show_html, {
    show_offcanvas(htmltools::HTML("<p>Rendered as an anonymous panel body.</p>"))
  })

  observeEvent(input$show_object, {
    show_offcanvas(offcanvas(
      "Rendered and shown by the server.",
      title = "New panel"
    ))
  })
}

shinyApp(ui, server)

The first button is the behavior introduced by this PR: it opens the predeclared existing-panel by id. The other two confirm that htmltools::HTML() continues to be body content and that passing an offcanvas() object is unchanged.

Breaking change

show_offcanvas("some string") used to create a new anonymous panel with that string as its body. It is now treated as an id lookup instead, and errors if the string looks like body text (contains whitespace, or is empty). This is a behavior change to a function released in bslib 0.12.0.

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