feat(theme-studio): add a real, exported :box face attribute

The mode-line box in the preview was hardcoded — it showed a box the generated theme couldn't actually produce, since build-theme.el never emitted :box. Made :box a real face attribute instead: a per-face box object (style line/raised/pressed, width, color) stored on UI and package faces, set from a "box" dropdown in both tables, rendered from the attribute everywhere (the mode-line bars, the package previews via ofs, the UI table preview cells), and exported through build-theme.el's --attrs as a proper :box plist (released/pressed → :style *-button; line → :line-width + optional :color). The hardcoded box is gone; mode-line and mode-line-inactive now default to the released-button box that is the Emacs default, so the preview and the export agree.

This also gives the package faces that genuinely use :box a way to represent it — the face audit found several (magit-branch-current/-remote-head, two flycheck list faces, the telega button family, ~15 slack button/dialog faces).

Tests: build-theme gains box-conversion + ui-box-emit ERT tests (24/24); the app-core deep-equal tests account for the new box slot; all 9 browser gates, 20 python, and 55 node tests stay green.

1 files changed, 21 insertions, 0 deletions

diff --git a/tests/test-build-theme.el b/tests/test-build-theme.el
index f74b9417..87b17e0a 100644
--- a/tests/test-build-theme.el
+++ b/tests/test-build-theme.el
@@ -192,6 +192,27 @@ mapping dec would clobber the type color."
     (should (member '(region ((t (:background "#264364")))) specs))
     (should (member '(mode-line ((t (:foreground "#cdced1" :background "#2f343a")))) specs))))
 
+(ert-deftest test-build-theme-box-styles ()
+  "Normal/Boundary: a face box spec converts to the right Emacs :box value."
+  (should (equal (build-theme/--box '((style . "released") (width . 1)))
+                 '(:line-width 1 :style released-button)))
+  (should (equal (build-theme/--box '((style . "pressed") (width . 2)))
+                 '(:line-width 2 :style pressed-button)))
+  (should (equal (build-theme/--box '((style . "line") (color . "#67809c")))
+                 '(:line-width 1 :color "#67809c")))
+  (should (equal (build-theme/--box '((style . "line"))) '(:line-width 1)))
+  (should (null (build-theme/--box nil)))
+  (should (null (build-theme/--box '((style . "none"))))))
+
+(ert-deftest test-build-theme-ui-face-emits-box ()
+  "Normal: a ui face with a box exports a :box attribute."
+  (let ((specs (build-theme/--ui-face-specs
+                '((mode-line . ((fg . "#cdced1") (bg . "#2f343a")
+                                (box . ((style . "released") (width . 1)))))))))
+    (should (member '(mode-line ((t (:foreground "#cdced1" :background "#2f343a"
+                                     :box (:line-width 1 :style released-button)))))
+                    specs))))
+
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