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, 2 insertions, 2 deletions

diff --git a/scripts/theme-studio/test-app-core.mjs b/scripts/theme-studio/test-app-core.mjs
index 9bf5145fa..162025254 100644
--- a/scripts/theme-studio/test-app-core.mjs
+++ b/scripts/theme-studio/test-app-core.mjs
@@ -62,7 +62,7 @@ test('buildPkgmap: Boundary — a face with no default dict still seeds blank',
   const m = buildPkgmap({ a: { faces: [['f', 'f']] } }, PAL);
   assert.deepEqual(m.a.f, {
     fg: null, bg: null, bold: false, italic: false, underline: false,
-    strike: false, inherit: null, height: 1, source: 'default',
+    strike: false, inherit: null, height: 1, box: null, source: 'default',
   });
 });
 
@@ -117,7 +117,7 @@ test('mergePackagesInto: Normal — fills missing fields with defaults', () => {
   mergePackagesInto(m, { a: { f: { fg: '#112233' } } });
   assert.deepEqual(m.a.f, {
     fg: '#112233', bg: null, bold: false, italic: false, underline: false,
-    strike: false, inherit: null, height: 1, source: 'user',
+    strike: false, inherit: null, height: 1, box: null, source: 'user',
   });
 });