Fix issues #1, #3, #5: misc improvements

- #5: Unwrap _brand.yml for Quarto 1.9+ compatibility (remove top-level
  brand: wrapper so meta/logo/color/typography are at the top level)
- #1: Add quiet parameter to na_sum() to suppress warnings in loops/pipelines
- #3: Add beep() function for CLI beep, desktop notifications, and webhook
  alerts (supports type=beep|notify|webhook|all)

tests/testthat/test_notifications.R

@@ -0,0 +1,22 @@
+# Test beep / notification function
+
+context("Test beep() function")
+
+test_that("beep() returns invisibly", {
+  expect_invisible(beep())
+  expect_invisible(beep("test", type = "beep"))
+  expect_invisible(beep("test", type = "all"))
+})
+
+test_that("beep() with quiet=TRUE produces no output", {
+  expect_silent(beep("test", type = "beep", quiet = TRUE))
+})
+
+test_that("beep() with type=notify is silent when no tools available", {
+  # When notify-send and osascript are both absent, should be silent
+  expect_silent(beep("test", type = "notify"))
+})
+
+test_that("beep() with type=webhook and no URL is silent", {
+  expect_silent(beep("test", type = "webhook"))
+})