USB label batch: brand colors + print-color-adjust; document migration strategy

- USBLabelBatch.vue: recolor controls/selection/cell borders to the app CSS
  variables (was hardcoded #667eea/#28a745/#dc3545) and add
  `print-color-adjust: exact` so barcodes print even with "Background graphics"
  off. (Barcodes render as vector SVG, so no canvas->img change needed.)
- ADR-004: document the resolved migration strategy - single core Alembic chain
  (`flask db upgrade`), bundled plugin schema folded into core (7c04), resolving
  the Phase 7B per-plugin-chain conflict footgun. External plugins may still
  ship their own chains.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/views/print/USBLabelBatch.vue

@@ -207,26 +207,27 @@ function print() {
 @page { size: letter; margin: 0; }
 
 .no-print { margin-bottom: 20px; padding: 20px; }
-.controls { background: white; padding: 20px; border-radius: 8px; margin-bottom: 20px; }
+.controls { background: var(--bg-card); color: var(--text); padding: 20px; border-radius: 8px; margin-bottom: 20px; border: 1px solid var(--border); }
 .controls h3 { margin-top: 0; }
 
 .print-btn {
   padding: 10px 30px;
   font-size: 16px;
   cursor: pointer;
-  background: #667eea;
+  background: var(--primary);
   color: white;
   border: none;
   border-radius: 5px;
   margin-right: 10px;
 }
-.print-btn:disabled { background: #ccc; cursor: not-allowed; }
+.print-btn:hover:not(:disabled) { background: var(--primary-dark); }
+.print-btn:disabled { background: var(--text-light); cursor: not-allowed; }
 
 .clear-btn {
   padding: 10px 20px;
   font-size: 14px;
   cursor: pointer;
-  background: #dc3545;
+  background: var(--danger);
   color: white;
   border: none;
   border-radius: 5px;
@@ -237,7 +238,7 @@ function print() {
   padding: 10px 20px;
   font-size: 14px;
   cursor: pointer;
-  background: #28a745;
+  background: var(--success);
   color: white;
   border: none;
   border-radius: 5px;
@@ -249,31 +250,32 @@ function print() {
   gap: 10px;
   max-height: 300px;
   overflow-y: auto;
-  border: 1px solid #ddd;
+  border: 1px solid var(--border);
   padding: 10px;
-  background: #fafafa;
+  background: var(--bg);
 }
 
 .usb-item {
   display: flex;
   align-items: center;
   padding: 8px;
-  background: white;
-  border: 1px solid #ddd;
+  background: var(--bg-card);
+  color: var(--text);
+  border: 1px solid var(--border);
   border-radius: 4px;
   cursor: pointer;
 }
-.usb-item:hover { background: #f0f0f0; }
-.usb-item.selected { background: #e7f1ff; border-color: #667eea; }
+.usb-item:hover { border-color: var(--primary); }
+.usb-item.selected { border-color: var(--primary); box-shadow: 0 0 0 1px var(--primary); }
 .usb-item input { margin-right: 10px; }
 .usb-item label { cursor: pointer; flex: 1; }
-.usb-item .alias { font-size: 11px; color: #666; }
+.usb-item .alias { font-size: 11px; color: var(--text-light); }
 
-.selected-count { font-weight: bold; margin: 10px 0; }
-.selected-count .count { color: #667eea; }
-.selected-count .pages { color: #28a745; }
+.selected-count { font-weight: bold; margin: 10px 0; color: var(--text); }
+.selected-count .count { color: var(--primary); }
+.selected-count .pages { color: var(--success); }
 
-.loading-msg { text-align: center; padding: 2rem; color: #666; }
+.loading-msg { text-align: center; padding: 2rem; color: var(--text-light); }
 
 .sheets-container { display: flex; flex-direction: column; gap: 20px; }
 
@@ -298,7 +300,7 @@ function print() {
   border: 1px dashed #ccc;
   overflow: hidden;
 }
-.label-cell.has-content { border: 1px solid #667eea; }
+.label-cell.has-content { border: 1px solid var(--primary); }
 .label-cell.empty { background: #fafafa; }
 
 .cell-1 { top: 0.875in; left: 1.1875in; }
@@ -353,6 +355,12 @@ function print() {
 }
 
 @media print {
+  /* Force the barcodes/borders to print even when "Background graphics" is
+     off. */
+  body, .print-sheet, .label-cell, .mini-label, .barcode-container {
+    -webkit-print-color-adjust: exact !important;
+    print-color-adjust: exact !important;
+  }
   body { padding: 0; margin: 0; background: white; }
   .no-print { display: none !important; }
   .sheets-container { gap: 0; }