bookmarks: column-based bookmark manager plugin

New plugin at public/plugins/bookmarks/ — bookmark lists stored as
cards tagged #bookmarks with a JSON body. Board cards render as
coloured tiles (70% card colour + surface) with a centred link icon,
title, and date/bucket metadata; no body preview.

Clicking opens a full-screen overlay with:
  - Named columns (click header to rename, right-click for
    rename + delete), + button to add columns
  - Bookmark items: favicon (stored as data: URI via server-side
    fetch-meta) + title with ellipsis. Click opens URL in new tab.
  - Right-click item → rename or delete
  - Positional drag-and-drop between and within columns — accent
    drop-indicator line shows the insertion point
  - Full-width slider sets relative column width, persisted per card
  - Horizontal wheel-to-scroll on the column container

Adding a bookmark reads the clipboard; if it contains a URL, the
backend fetches <title> + favicon (as base64 data: URI, ≤32 KB) so
the card is fully self-contained. Non-URL clipboard → manual entry
dialog. Also accepts card:123 links — rendered with a card icon in
the linked card's colour; click opens the linked card's detail view
or, if it's another bookmark list, switches the overlay to it.

Multiple bookmark lists supported: header button always shows a
picker (even with one list) with a "new list" option. New lists
inherit the active bucket. Overlay title is click-to-rename.

Tests: plugin registration, header button injection, board card
decoration, overlay open/close + column + item rendering, fetch-meta
API shape, card-link icon rendering, column-width persistence.

Also fixes the flaky undo due_date test — replaced a fixed sleep
with waitForStateCard polling so the undo PATCH has time to settle.

Bumps to 1.9.23.

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

public/plugins/bookmarks/bookmarks.html

@@ -0,0 +1,19 @@
+<!-- Bookmarks overlay. Opens when the user clicks a #bookmarks card
+     on the board (instead of the normal detail view). Column-based
+     layout; each column is a topic, each item is a bookmark with
+     favicon + title. -->
+<div id="bookmarks-overlay" class="overlay fixed inset-0 z-[80]" style="background:var(--t-overlay)">
+    <div class="bookmarks-shell">
+        <div class="bookmarks-header">
+            <i class="ph ph-link text-t-accent text-base"></i>
+            <span id="bookmarks-title" class="font-pixel text-sm text-t-accent flex-1 truncate cursor-pointer hover:text-t-bright transition-colors" data-tip="click to rename"></span>
+            <button id="bookmarks-add-btn" class="bm-btn" data-tip="new bookmark (paste URL)"><i class="ph ph-plus text-sm"></i></button>
+            <button id="bookmarks-col-add" class="bm-btn" data-tip="new column"><i class="ph ph-columns-plus-right text-sm"></i></button>
+            <button id="bookmarks-close" class="bm-btn hover:text-t-danger"><i class="ph ph-x text-sm"></i></button>
+        </div>
+        <div class="bookmarks-slider-row">
+            <input type="range" id="bookmarks-col-width" min="140" max="400" step="10" value="220" class="bookmarks-slider">
+        </div>
+        <div id="bookmarks-columns" class="bookmarks-columns"></div>
+    </div>
+</div>