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/components/ConfirmDialog.vue

@@ -0,0 +1,37 @@
+<script setup lang="ts">
+withDefaults(
+  defineProps<{
+    title: string;
+    message: string;
+    confirmLabel?: string;
+    danger?: boolean;
+  }>(),
+  {
+    confirmLabel: "Delete",
+    danger: true,
+  }
+);
+
+const emit = defineEmits<{
+  confirm: [];
+  cancel: [];
+}>();
+</script>
+
+<template>
+  <teleport to="body">
+    <div class="modal-overlay" @click="emit('cancel')">
+      <div class="modal-card" @click.stop>
+        <h3 class="modal-title">{{ title }}</h3>
+        <p class="modal-message">{{ message }}</p>
+        <div class="modal-actions">
+          <button class="modal-btn" @click="emit('cancel')">Cancel</button>
+          <button
+            :class="['modal-btn', { 'modal-btn-danger': danger }]"
+            @click="emit('confirm')"
+          >{{ confirmLabel }}</button>
+        </div>
+      </div>
+    </div>
+  </teleport>
+</template>