v0.5.3: dark-mode branding preview + unified button spacing

UI polish:
- Settings → Branding: each logo swatch now previews on a background
  matching where the mark lands (light page / dark page / dark PXE screen)
  regardless of the current page theme, so the Dark slot reads as dark
  even while viewing Settings in light mode.
- Site-wide button spacing: add one rule (`.card .body > button`) giving
  every primary card action button the same gap above it, and drop the
  ad-hoc per-button inline margins (14/16/6px) so the look is uniform.
  Fixes the Hosts → "Bind MAC to target" button butting against the form.

(Boot-menu highlight intentionally unchanged — a rotating-RGB highlight
isn't possible in iPXE's static single-draw menu; deferred to a future
custom-renderer effort.)

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

crates/webui/src/app.css

@@ -304,6 +304,14 @@ button.ghost { background: transparent; color: var(--fg); border: 1px solid var(
 button.ghost:hover { background: var(--bg-panel-2); color: var(--fg); }
 button.danger { background: transparent; color: var(--err); border: 1px solid color-mix(in srgb, var(--err) 35%, transparent); }
 button.danger:hover { background: color-mix(in srgb, var(--err) 14%, transparent); color: var(--err); }
+/* v0.5.3: unified spacing for a card's primary action button(s). Any
+   button that sits as a direct child of a card body (Save, Bind, Add,
+   Launch, …) gets the same gap above it so it never butts against the
+   form. Inline buttons inside table rows / toolbars / logo slots /
+   modal action bars are nested deeper, so the `>` keeps them untouched.
+   Adjacent action buttons on one row (e.g. Save + Send test) share the
+   margin and stay aligned. */
+.card .body > button { margin-top: 16px; }
 
 label.field {
   display: grid; gap: 4px; margin-bottom: 14px;