feat(design): surface phase PR 4 — Knowledge cluster polish

Per the surface-phase spec for the Notes/Tasks viewers and editors:

Long-form line-height
- prose.css: bumped global .prose from 1.6 to 1.7. Applies to Note
  viewer body, Task viewer body, anywhere markdown renders into a
  reading surface. Chat assistant bubble already had the explicit
  override; now consistent with the rest.

Button reclassification per Hybrid rule
- Shared editor-shared.css:
  - btn-save: accent gradient → Moss (action-primary). Saving is
    "operating the software", not a brand moment.
  - btn-delete: --color-danger (Error terracotta) → Oxblood
    (action-destructive). Layout updated for inline-flex so the
    Trash2 icon at call sites lines up alongside the label.
- NoteEditorView, TaskEditorView: Delete buttons now contain a
  Trash2 icon per Hybrid's "destructive paired with icon" rule.
- NoteViewerView, TaskViewerView:
  - btn-edit (and TaskViewer's btn-advance): accent gradient → Moss.
    Switching to edit / advancing status are workflow actions.
  - btn-convert, btn-share: ghost-on-hover-to-accent → Bronze
    action-secondary (alternate paths).

Two-weights-only
- Snapped every font-weight: 600/700 to 500 across editor-shared.css
  and the five Knowledge-cluster views.

Out of scope for this PR (deliberate punt)
- Smaller utility buttons (.btn-suggest-tags, .btn-link-all,
  .btn-add-subtask, the AI-assist generate/proofread/accept/reject
  set, etc.) — currently ghost-styled, generally compliant. Will
  revisit only if they read off in practice.
- Filter chip / sub-task list-row border audit — deferred since
  current styles already lean on background tint for affordance.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/views/NoteEditorView.vue

@@ -21,6 +21,7 @@ import MilestoneSelector from "@/components/MilestoneSelector.vue";
 import DiffView from "@/components/DiffView.vue";
 import VersionHistorySection from "@/components/VersionHistorySection.vue";
 import ConfirmDialog from "@/components/ConfirmDialog.vue";
+import { Trash2 } from "lucide-vue-next";
 
 const route = useRoute();
 const router = useRouter();
@@ -449,7 +450,9 @@ onUnmounted(() => assist.clearSelection());
         <button class="btn-save" @click="save" :disabled="saving">
           {{ saving ? "Saving..." : "Save" }}
         </button>
-        <button v-if="isEditing" class="btn-delete" @click="remove">Delete</button>
+        <button v-if="isEditing" class="btn-delete" @click="remove">
+          <Trash2 :size="16" /> Delete
+        </button>
         <WordCount :body="body" />
       </div>
       <input
@@ -1013,7 +1016,7 @@ onUnmounted(() => assist.clearSelection());
 
 .assist-section-title {
   font-size: 0.78rem;
-  font-weight: 700;
+  font-weight: 500;
   color: var(--color-text-secondary);
   text-transform: uppercase;
   letter-spacing: 0.05em;