super-buckets, dnd insert cursor for cards + buckets, drag-state survives polls

Super-buckets: a bucket can act as a dropdown of nested children, 1-level
deep, created by dragging one bucket onto the center of another. The
parent shows up as a single tile in the bar with a stack icon and the
active member's name; clicking opens a popover that lists parent + all
children. Selecting an entry sets state.activeBucket. Implementation:

* src/Database.php — ALTER TABLE buckets ADD COLUMN parent_id + index.
* public/index.php — PATCH /api/buckets accepts parent_id with depth /
  cycle / collab guards. DELETE on a parent un-nests its children
  rather than cascade-deleting. The /api/import handler does a second
  pass to remap parent_id through the new bucket id map so backup +
  restore round-trips the hierarchy.
* public/js/modules/buckets.js renderBar() — top-level buckets only;
  buckets with ≥1 child render as a `.bucket-super` tile with
  data-bucket-super=<parent.id> and data-super-shown=<active member>.
  Three drop zones per tile: left/right 25% → reorder via insert
  cursor, center 50% → nest (PATCH parent_id) with .bucket-nest-target
  outline. canNestInto() mirrors the backend rules client-side.
  openSuperDropdown() renders a popover under the tile with parent +
  children as real .bucket-tab[data-bucket=…] entries (so the collab
  plugin's MutationObserver still picks them up); appended inside
  #bucket-bar but `position:fixed` so the bar's overflow:auto can't
  clip it. showBucketCtx() now scopes move-left/right to siblings
  (top-level peers OR same parent_id) so the action works in both
  the bar and the dropdown.

Drag-and-drop insertion cursor for cards (matches the bucket fix):

* public/js/modules/board.js bindCardEvents() — single
  #card-drop-cursor element inside #board, repositioned by per-card
  dragover. Three zones (left/right 25% → cursor in gap, center 50% →
  card-drop-group highlight). dragleave does NOT hide the cursor (was
  flickering as the mouse moved between adjacent cards); only dragend
  clears it. handleCardDrop recomputes the drop zone from e.clientX
  since the cursor approach no longer tags the target with
  .card-drop-before / .card-drop-after.
* "Vor der ersten Card" works automatically: the first card's left
  25% positions the cursor at offsetLeft - 6, which sits inside
  #board's px-6 padding and stays visible.
* public/css/style.css — old .card-drop-before::before /
  .card-drop-after::after pseudo-elements removed in favour of the
  #card-drop-cursor rule.

Drag ghost survives background polls:

* board.js draggingCardIds: Set<string> at module scope. dragstart
  fills it; dragend clears. renderBoard() re-applies the
  .card-dragging class to every member of the set after every render
  so caldav / collab polls calling _reloadAll mid-drag don't
  visually pop the source card back to its normal state.

Tests (public/plugins/tests/super-buckets.test.js):

* API parent_id: nest, un-nest, self-parent rejected, level-2
  rejected (bucket with own children, target is itself a child),
  DELETE un-nests, export → import preserves parent_id.
* Bar rendering: super-tile DOM markers, label = parent name when
  no child active, label = active child name when selected, popover
  opens with parent + children entries, click child sets activeBucket,
  click super-tile twice toggles popover closed, plain bucket stays
  plain.
* Drop cursor element exists.
* Both describe blocks reset state.activeBucket = 'all' in afterEach
  — the rendering tests set it to a child id and the leak was breaking
  downstream Card: Due Date & Done / Card: Grouping tests by filtering
  every newly-created card with bucket_id=null off the board.

chore: bump version to 0.9.3

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/css/style.css

@@ -426,21 +426,25 @@ body::after {
     pointer-events: none; z-index: 10;
 }
 
-/* Drop indicators — thick glowing bar */
-.card-drop-before::before,
-.card-drop-after::after {
-    content: '';
+/* Card drop cursor — single absolutely-positioned vertical line that
+   lives inside #board (which is `position: relative`) and gets
+   repositioned by the dragover handlers in `board.js`. Replaces the
+   old `.card-drop-before::before` / `.card-drop-after::after` pseudo-
+   elements so the indicator is a real DOM node in the gap between cards
+   instead of a sliver glued to one card. The "before first card"
+   case works automatically: the first card's left-25% zone positions
+   the cursor at `card.offsetLeft - 6`, which sits in #board's left
+   padding and stays visible. */
+#card-drop-cursor {
     position: absolute;
-    top: 8px;
-    bottom: 8px;
     width: 4px;
     background: var(--t-accent);
-    border-radius: 2px;
     box-shadow: 0 0 12px var(--t-accent), 0 0 4px var(--t-accent);
+    border-radius: 2px;
+    pointer-events: none;
     z-index: 10;
+    display: none;
 }
-.card-drop-before::before { left: -6px; }
-.card-drop-after::after { right: -6px; }
 
 /* Group fan preview */
 .group-fan {
@@ -665,8 +669,47 @@ body.detail-image-fs-open { overflow: hidden; }
 }
 .bucket-add.drag-over i,
 .bucket-add.drag-over span { color: inherit !important; }
-.bucket-drop-left { border-left: 3px solid var(--t-accent) !important; }
-.bucket-drop-right { border-right: 3px solid var(--t-accent) !important; }
+/* Bucket reorder drop cursor — a single absolutely-positioned vertical
+   line that lives inside #bucket-bar and gets repositioned by the
+   dragover handler in `public/js/modules/buckets.js`. Showing the cursor
+   *between* tiles makes the insert point unambiguous next to the
+   super-bucket nest gesture (center 50% → nest, edges 25% → reorder). */
+#bucket-drop-cursor {
+    position: absolute;
+    top: 4px;
+    bottom: 4px;
+    width: 3px;
+    background: var(--t-accent);
+    box-shadow: 0 0 6px color-mix(in srgb, var(--t-accent) 70%, transparent);
+    border-radius: 1px;
+    pointer-events: none;
+    z-index: 4;
+    display: none;
+}
+/* Drop-into-center highlight: target tile gets an outline that says
+   "release here to nest". Distinct from the insertion cursor so the
+   user can tell reorder vs. nest at a glance. */
+.bucket-tab.bucket-nest-target {
+    outline: 2px dashed var(--t-accent) !important;
+    outline-offset: 1px;
+    background: color-mix(in srgb, var(--t-accent) 12%, transparent) !important;
+}
+
+/* Super-bucket dropdown popover. Positioned via inline styles by
+   openSuperDropdown() in buckets.js. Lives on document.body (not
+   inside #bucket-bar) so it can escape the bar's overflow:auto. */
+#bucket-super-popover {
+    z-index: 220;
+    background: var(--t-surface);
+    border: 1px solid var(--t-border);
+    border-radius: var(--ui-radius-modal);
+    box-shadow: var(--ui-shadow-modal, 0 10px 30px rgba(0,0,0,0.4));
+    padding: 4px;
+    min-width: 180px;
+    display: flex;
+    flex-direction: column;
+    gap: 2px;
+}
 
 /* Tag chips */
 .tag-chip {