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/KnowledgeView.vue

@@ -817,7 +817,7 @@ onUnmounted(() => {
   color: #fff;
   cursor: pointer;
   font-size: 0.85rem;
-  font-weight: 600;
+  font-weight: 500;
   transition: box-shadow 0.15s;
 }
 .btn-new-note:hover { box-shadow: var(--glow-cta-hover); }
@@ -1067,7 +1067,7 @@ onUnmounted(() => {
   font-size: 0.68rem;
   padding: 2px 7px;
   border-radius: 10px;
-  font-weight: 600;
+  font-weight: 500;
   text-transform: uppercase;
   letter-spacing: 0.04em;
 }
@@ -1079,7 +1079,7 @@ onUnmounted(() => {
 
 .k-card-body { flex: 1; padding-right: 40px; }
 .k-card-title {
-  font-weight: 600;
+  font-weight: 500;
   font-size: 0.92rem;
   margin-bottom: 5px;
   line-height: 1.3;
@@ -1193,7 +1193,7 @@ onUnmounted(() => {
   font-size: 0.7rem;
   padding: 1px 7px;
   border-radius: 8px;
-  font-weight: 600;
+  font-weight: 500;
 }
 .status--todo { background: var(--color-status-todo-bg); color: var(--color-status-todo); }
 .status--in_progress { background: var(--color-status-in-progress-bg); color: var(--color-status-in-progress); }
@@ -1204,7 +1204,7 @@ onUnmounted(() => {
   font-size: 0.7rem;
   padding: 1px 7px;
   border-radius: 8px;
-  font-weight: 600;
+  font-weight: 500;
 }
 .priority--low { background: var(--color-priority-low-bg); color: var(--color-priority-low); }
 .priority--normal { background: var(--color-priority-medium-bg); color: var(--color-priority-medium); }
@@ -1272,7 +1272,7 @@ onUnmounted(() => {
   justify-content: space-between;
   padding: 10px 14px;
   font-size: 0.85rem;
-  font-weight: 600;
+  font-weight: 500;
   border-bottom: 1px solid var(--color-border, rgba(255,255,255,0.06));
   flex-shrink: 0;
 }
@@ -1335,7 +1335,7 @@ onUnmounted(() => {
 }
 .minichat-title {
   font-size: 0.82rem;
-  font-weight: 600;
+  font-weight: 500;
   color: var(--color-muted);
   text-transform: uppercase;
   letter-spacing: 0.06em;