Use g6R instead of DiagrammeR for dm_draw()

[christophsax]

Motivation

dm_draw() currently uses DiagrammeR/Graphviz for rendering ER diagrams. While functional, there are reasons to consider g6R as an alternative:

1. Lighter dependencies

For typical dm users (with shiny for dm_gui()):

• DiagrammeR adds 25 packages including readr, vroom, visNetwork, scales
• g6R adds only 8 packages (knitr/rmarkdown - most users already have these)

2. Modern styling

• Full HTML/CSS control for node appearance
• Proper table-like cards with bold headers, shadows, rounded corners
• PK/FK markers (🔑/→) clearly visible

3. Native interactivity

• Built-in pan/zoom/drag without hacks
• dm_gui() currently requires SVG export + regex onclick injection
• g6R widgets have native Shiny input bindings

4. Ecosystem consistency

• g6R is already used in blockr.dag
• Consistent visualization across related packages

Implementation Plan

Phase 1: Add g6R as optional backend

• New backend = "g6R" option for dm_draw()
• Keep DiagrammeR as default for backward compatibility
• Support existing options: view_type, rankdir, colors

Phase 2: Migrate dm_gui()

• Replace SVG export approach with native g6R widget
• Cleaner code, better interactivity

Phase 3: Make g6R default

• Deprecate DiagrammeR backend
• Remove DiagrammeR from Suggests

Preview

With g6R backend and rankdir = "TB":

dm_nycflights13() |> dm_draw(backend = "g6R", rankdir = "TB")

[christophsax]

In blockr.dm, I have now a more 'tabular' dm visualization, which fits better into blockr: https://blockr.cloud/app/duckdb-remote-dm

This has no special dependencies, no DiagrammeR, no g6r.

Comments very welcome. If you like I could contribute an updated dm_draw.