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.

0 files changed, 0 insertions, 0 deletions