DRY refactoring pass: shared mixins, route helpers, composables, CSS

Backend:
- models/base.py: TimestampMixin + CreatedAtMixin; applied to all 10+ models
- routes/utils.py: not_found() + parse_iso_date() helpers; used across all route files
- routes/milestones.py: _milestone_dict() helper replaces 5 repeated to_dict + progress blocks

Frontend:
- 5 new composables: useAutoSave, useEditorGuards, useTagSuggestions,
  useFloatingAssist, useListKeyboardNavigation
- ConfirmDialog.vue: reusable confirm modal replacing inline <teleport> blocks
- editor-shared.css: sidebar CSS consolidated from both editor views
- viewer-shared.css: context-bar/breadcrumb CSS consolidated from both viewer views
- NoteEditorView + TaskEditorView: ~120 lines each replaced with composable calls;
  duplicate scoped sidebar CSS removed
- NotesListView + TasksListView: inline keyboard-nav replaced with composable
- NoteViewerView + TaskViewerView: duplicate context-bar CSS removed

No behaviour changes. Net: -634 lines, +237 lines across 31 files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/assets/editor-shared.css

@@ -532,6 +532,70 @@
   pointer-events: auto;
 }
 
+/* ── Sidebar shared styles ── */
+.sidebar-toggle {
+  display: none;
+  width: 100%;
+  padding: 0.6rem 1rem;
+  background: var(--color-bg-secondary);
+  border: none;
+  border-bottom: 1px solid var(--color-border);
+  font-size: 0.85rem;
+  font-weight: 600;
+  color: var(--color-text-secondary);
+  cursor: pointer;
+  text-align: left;
+  font-family: inherit;
+}
+.sidebar-content {
+  padding: 0.75rem;
+  display: flex;
+  flex-direction: column;
+  gap: 0.65rem;
+}
+.sb-field {
+  display: flex;
+  flex-direction: column;
+  gap: 0.25rem;
+}
+.sb-label {
+  font-size: 0.78rem;
+  font-weight: 600;
+  color: var(--color-text-secondary);
+  text-transform: uppercase;
+  letter-spacing: 0.04em;
+}
+.sb-select,
+.sb-input {
+  width: 100%;
+  padding: 0.35rem 0.5rem;
+  border: 1px solid var(--color-input-border);
+  border-radius: var(--radius-sm);
+  background: var(--color-bg-card);
+  color: var(--color-text);
+  font-size: 0.875rem;
+  font-family: inherit;
+  box-sizing: border-box;
+}
+.sb-select:focus,
+.sb-input:focus {
+  outline: none;
+  border-color: var(--color-primary);
+}
+.sb-divider {
+  height: 1px;
+  background: var(--color-border);
+  margin: 0.15rem 0;
+}
+@media (max-width: 720px) {
+  .sidebar-toggle { display: block; }
+  .sidebar-content {
+    display: none;
+    padding: 0.75rem 1rem;
+  }
+  .sidebar-content.sidebar-open { display: flex; }
+}
+
 /* ── Mobile ── */
 @media (max-width: 768px) {
   .editor-body {