feat(music): add playlist UI with header overlay, track styling, and tests

Replace raw file paths with clean track descriptions (Artist - Title [M:SS]),
add multi-line header overlay showing playlist info, now-playing, mode
indicators with gold/muted toggle states, and keybinding hints. Style
non-playing tracks in Dupre steel, playing track in Dupre gold. Playlist
window now opens from the bottom at 50% height with subtle background
highlight on focus.

Unit tests for format-duration (13 tests) and track-description (16 tests).

1 files changed, 91 insertions, 0 deletions

diff --git a/tests/test-music-config--format-duration.el b/tests/test-music-config--format-duration.el
new file mode 100644
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+++ b/tests/test-music-config--format-duration.el
@@ -0,0 +1,91 @@
+;;; test-music-config--format-duration.el --- Tests for duration formatting -*- coding: utf-8; lexical-binding: t; -*-
+;;
+;; Author: Craig Jennings <c@cjennings.net>
+;;
+;;; Commentary:
+;; Unit tests for cj/music--format-duration function.
+;; Tests the pure helper that converts seconds to "M:SS" display strings.
+;;
+;; Test organization:
+;; - Normal Cases: Typical durations, exact minutes, seconds only
+;; - Boundary Cases: Zero, one second, large values, float input
+;; - Error Cases: Nil, negative, non-numeric input
+;;
+;;; Code:
+
+(require 'ert)
+
+;; Stub missing dependencies before loading music-config
+(defvar-keymap cj/custom-keymap
+  :doc "Stub keymap for testing")
+
+;; Add EMMS elpa directory to load path for batch testing
+(let ((emms-dir (car (file-expand-wildcards
+                      (expand-file-name "elpa/emms-*" user-emacs-directory)))))
+  (when emms-dir
+    (add-to-list 'load-path emms-dir)))
+
+(require 'emms)
+(require 'emms-playlist-mode)
+(require 'music-config)
+
+;;; Normal Cases
+
+(ert-deftest test-music-config--format-duration-normal-typical-song ()
+  "Validate typical 3:45 song duration."
+  (should (string= (cj/music--format-duration 225) "3:45")))
+
+(ert-deftest test-music-config--format-duration-normal-exact-minutes ()
+  "Validate exact minute boundary produces M:00."
+  (should (string= (cj/music--format-duration 180) "3:00")))
+
+(ert-deftest test-music-config--format-duration-normal-seconds-only ()
+  "Validate sub-minute duration produces 0:SS."
+  (should (string= (cj/music--format-duration 45) "0:45")))
+
+(ert-deftest test-music-config--format-duration-normal-single-digit-seconds ()
+  "Validate seconds are zero-padded to two digits."
+  (should (string= (cj/music--format-duration 62) "1:02")))
+
+(ert-deftest test-music-config--format-duration-normal-long-track ()
+  "Validate duration over 10 minutes."
+  (should (string= (cj/music--format-duration 622) "10:22")))
+
+;;; Boundary Cases
+
+(ert-deftest test-music-config--format-duration-boundary-one-second ()
+  "Validate minimum positive duration."
+  (should (string= (cj/music--format-duration 1) "0:01")))
+
+(ert-deftest test-music-config--format-duration-boundary-zero-returns-nil ()
+  "Validate zero seconds returns nil (no useful duration)."
+  (should (null (cj/music--format-duration 0))))
+
+(ert-deftest test-music-config--format-duration-boundary-very-large ()
+  "Validate very long duration (over an hour) formats correctly."
+  (should (string= (cj/music--format-duration 3661) "61:01")))
+
+(ert-deftest test-music-config--format-duration-boundary-float-input ()
+  "Validate float seconds are truncated by integer division."
+  (should (string= (cj/music--format-duration 125.7) "2:05")))
+
+(ert-deftest test-music-config--format-duration-boundary-59-seconds ()
+  "Validate max seconds before minute rollover."
+  (should (string= (cj/music--format-duration 59) "0:59")))
+
+;;; Error Cases
+
+(ert-deftest test-music-config--format-duration-error-nil-returns-nil ()
+  "Validate nil input returns nil."
+  (should (null (cj/music--format-duration nil))))
+
+(ert-deftest test-music-config--format-duration-error-negative-returns-nil ()
+  "Validate negative input returns nil."
+  (should (null (cj/music--format-duration -5))))
+
+(ert-deftest test-music-config--format-duration-error-string-returns-nil ()
+  "Validate non-numeric input returns nil."
+  (should (null (cj/music--format-duration "3:45"))))
+
+(provide 'test-music-config--format-duration)
+;;; test-music-config--format-duration.el ends here