Rename amRshiny references to amRviz

[eboyer221]

#Description
The amRshiny package has been renamed to amRviz. This PR updates all references to the old name across the amR package suite.

##Changes

• DESCRIPTION - updated title and description text
• R/install_amR.R - updated roxygen docs; renamed the packages argument value "shiny" → "viz"
• updated valid_packages and repo_map to point at JRaviLab/amRviz
• man/installAMR.Rd - regenerated to match
• README.Rmd / README.md - updated install commands, quick-start, workflow diagram, and documentation links
• vignettes/intro.Rmd - updated package list and workflow steps
• tests/testthat/test-install.R - updated expected defaults and valid-package vectors

##Note to Users
installAMR(packages = "shiny") no longer works; use installAMR(packages = "viz"). Users who call installAMR() with no arguments are unaffected, since the default already includes "viz".

##To Test

• devtools::check() passes (0 errors / 0 warnings / 0 notes)
• devtools::test() passes
• installAMR() with defaults pulls JRaviLab/amRviz (not amRshiny)
• installAMR(packages = "viz") works; installAMR(packages = "shiny") errors as expected
• README on GitHub shows amRviz everywhere

[jananiravi]

Emily, please make sure the replacement image is at least 600 dpi. The rest of it looks good -- seems straightforward!