Add multi-select and bulk actions to the Library tab

- Checkboxes on every row/tile plus a "Select all" (filtered-set-aware,
  indeterminate-aware) toggle in the toolbar.
- A bulk-action bar appears once anything's selected: add to a deck
  ("Add 1" sets each selected card to 1 copy, "Add All" sets it to
  however many you own — the latter is the one-click fix for bulk-adding
  a stack of basic lands), set location, and adjust owned quantity by a
  configurable delta. All reuse the same per-card commands the
  single-card UI already calls, so behavior stays consistent (including
  the existing grace-period undo on quantity_owned hitting 0).
- CardTile gains a `cornerLeft` slot and `selected` prop (ring highlight,
  keeps the checkbox visible without hovering) to support this in tile
  view without disturbing the existing top-right qty-stepper corner.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/styles.css

@@ -203,6 +203,13 @@ select option {
   color: var(--text);
 }
 
+input[type="checkbox"] {
+  width: 15px;
+  height: 15px;
+  accent-color: var(--accent);
+  cursor: pointer;
+}
+
 /* ============================================================
    App shell: nav rail + main
    ============================================================ */
@@ -950,6 +957,39 @@ tbody tr:hover {
   margin-right: 4px;
 }
 
+/* ============================================================
+   Bulk-selection action bar (Library tab)
+   ============================================================ */
+
+.bulk-bar {
+  display: flex;
+  align-items: center;
+  gap: 16px;
+  flex-wrap: wrap;
+  background: var(--accent-subtle);
+  border: 1px solid var(--accent-glow);
+  border-radius: var(--radius-lg);
+  padding: 10px 16px;
+  margin-bottom: 16px;
+}
+.bulk-bar-count {
+  font-weight: 700;
+  font-size: 13px;
+  color: var(--accent-hover);
+  white-space: nowrap;
+}
+.bulk-bar-actions {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  flex-wrap: wrap;
+}
+.bulk-bar-delta {
+  width: 48px;
+  text-align: center;
+  padding: 8px 4px;
+}
+
 /* ============================================================
    Filters panel
    ============================================================ */
@@ -1158,6 +1198,34 @@ tbody tr:hover {
 .tile:focus-within .tile-corner {
   opacity: 1;
 }
+.tile-corner-left {
+  position: absolute;
+  top: 8px;
+  left: 8px;
+  /* Same reasoning as .tile-corner: don't cover the card name unless the
+     tile is being interacted with — except once selected, where the
+     checkbox should stay visible so the selection is visible at a glance. */
+  opacity: 0;
+  transition: opacity 0.12s;
+}
+.tile:hover .tile-corner-left,
+.tile:focus-within .tile-corner-left,
+.tile-selected .tile-corner-left {
+  opacity: 1;
+}
+.tile-selected .tile-img {
+  box-shadow: 0 0 0 3px var(--accent), var(--shadow-md);
+}
+.tile-select {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 26px;
+  height: 26px;
+  border-radius: 999px;
+  background: rgba(0, 0, 0, 0.78);
+  backdrop-filter: blur(2px);
+}
 .tile-badge {
   background: rgba(0, 0, 0, 0.78);
   color: #fff;