shortcuts: central registry + auto-generated help, test stabilization

New module public/js/modules/shortcuts.js:
* registerShortcut({key, ctrl, shift, alt, match, guard, priority,
  section, label, keys, handler}) — declarative shortcut registration
  with priority ordering, custom match predicates, and runtime guards.
* Single document-level keydown dispatcher iterates the registry
  (sorted by priority desc), matches key+modifiers or custom match(),
  checks guard(), fires handler with preventDefault.
* guards object: board(), editor(), detail(), search(), email(),
  canvas(), bodyFocused(), noOverlay(), isOpen(id) — reusable
  predicates that replace the repeated blocker-array patterns.
* renderShortcutHelp() auto-generates the #shortcuts overlay content
  from the registry, grouped by section. New shortcuts added via
  registerShortcut appear in the help screen automatically.
* Shortcuts with guard: () => false are help-screen-only entries for
  element-level shortcuts handled by their own modules (editor
  textarea, search input, canvas overlay, etc.).

Migration (public/js/app.js):
* The ~200-line monolithic keydown handler is replaced by ~80
  declarative registerShortcut() calls.
* Escape chain: 10 separate registrations with priorities 100→20,
  each with its own guard (quick-capture, detail/zen, modal, search,
  tasks, gallery, calendar, shortcuts, help). A hidden: true flag
  suppresses them from the help screen; one visible entry with
  guard: () => false shows "Esc — close any overlay".
* Global combos: Ctrl+K (search), Ctrl+N (quick capture), Ctrl+Z
  (undo), F1 (help), ? (shortcuts).
* Editor: Ctrl+S (save), Ctrl+Shift+S (save+close), Ctrl+Enter
  (save+close). Element-level shortcuts (Tab indent, Alt+arrows,
  autocomplete triggers) registered as help-screen-only.
* Board bare keys: a/f/j/n/t/c//, Tab/Shift+Tab, 1-9, arrow nav,
  Enter → preview, Delete/Backspace selected.
* Detail: ←/→ group navigation, Enter → editor.
* Help-screen-only: search ↑↓/Enter, board mouse actions, canvas
  tool shortcuts (B/E/L/R/O/F/T/⌫), mail ⌫/⇧⌫, m (plugin).

Removed from board.js: the document-level Delete/Backspace listener
(now in the central registry with the same logic).

Help screen (public/index.php):
* The hardcoded 3-column shortcut HTML is replaced by an empty
  .shortcut-help-content div that renderShortcutHelp() fills at boot.
* CSS: .shortcut-help-content section gets break-inside:avoid +
  margin-bottom for clean column flow.

Test stabilization:
* card-interactions.test.js: due-pill, overdue-border, and
  child-hidden-on-board tests now set state.activeBucket='all'
  before _reloadAll and poll for the DOM element (40×50ms) instead
  of a fixed sleep(200). Eliminates the flaky "card not found" when
  activeBucket was leaked by a prior test.
* editor-features.test.js: "save creates card and closes editor"
  polls for #modal to lose .open class (40×50ms) instead of a fixed
  sleep(400). doSave(true) does 2-3 serial API calls before
  closeModal which sometimes exceeded the old budget.

chore: bump version to 0.9.13

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

public/css/style.css

@@ -838,11 +838,15 @@ body.anim-slow .board-card.card-pulse-new {
     font-family:var(--t-font-pixel, 'Silkscreen', cursive);
     font-size:9px; color:var(--t-dim);
 }
+/* The auto-generated shortcut help uses a 3-column grid on the
+   .shortcut-help-content div. Collapse to 2 then 1 on smaller screens. */
+.shortcut-help-content { column-gap: 20px; }
+.shortcut-help-content section { break-inside: avoid; margin-bottom: 16px; }
 @media (max-width: 900px) {
-    #shortcuts [style*="grid-template-columns"] { grid-template-columns: 1fr 1fr !important; }
+    .shortcut-help-content { grid-template-columns: 1fr 1fr !important; }
 }
 @media (max-width: 640px) {
-    #shortcuts [style*="grid-template-columns"] { grid-template-columns: 1fr !important; }
+    .shortcut-help-content { grid-template-columns: 1fr !important; }
 }
 
 /* Tag chips */