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/views/NoteViewerView.vue

@@ -229,6 +229,7 @@ async function convertToTask() {
   </div>
 </template>
 
+<style src="@/assets/viewer-shared.css" />
 <style scoped>
 .viewer-layout {
   display: flex;
@@ -294,48 +295,6 @@ async function convertToTask() {
   cursor: default;
 }
 
-/* Context breadcrumb */
-.context-bar {
-  display: flex;
-  align-items: center;
-  gap: 0.4rem;
-  flex-wrap: wrap;
-  margin-bottom: 0.6rem;
-}
-.ctx-crumb {
-  display: inline-flex;
-  align-items: center;
-  font-size: 0.8rem;
-  padding: 0.15rem 0.55rem;
-  border-radius: 999px;
-  white-space: nowrap;
-}
-.ctx-crumb-parent {
-  color: var(--color-text-muted);
-  background: var(--color-bg-secondary);
-  border: 1px solid var(--color-border);
-  text-decoration: none;
-}
-.ctx-crumb-parent:hover {
-  color: var(--color-primary);
-  border-color: var(--color-primary);
-}
-.ctx-crumb-project {
-  color: var(--color-primary);
-  background: color-mix(in srgb, var(--color-primary) 10%, transparent);
-  border: 1px solid color-mix(in srgb, var(--color-primary) 30%, transparent);
-  text-decoration: none;
-  font-weight: 500;
-}
-.ctx-crumb-project:hover {
-  background: color-mix(in srgb, var(--color-primary) 18%, transparent);
-}
-.ctx-crumb-milestone {
-  color: var(--color-text-secondary);
-  background: var(--color-bg-secondary);
-  border: 1px solid var(--color-border);
-}
-
 .meta {
   font-size: 0.85rem;
   color: var(--color-text-muted);