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/NoteViewerView.vue

@@ -345,41 +345,39 @@ async function convertToTask() {
   border-color: var(--color-primary);
   color: var(--color-primary);
 }
+/* Edit: Moss action-primary — switching from view to edit is operating
+   the software, not a brand moment. */
 .btn-edit {
   display: inline-flex;
   align-items: center;
   padding: 0.45rem 1.1rem;
   border: none;
   border-radius: var(--radius-sm);
-  background: var(--gradient-cta);
+  background: var(--color-action-primary);
   color: #fff;
   text-decoration: none;
   cursor: pointer;
   font-size: 0.875rem;
-  font-weight: 600;
-  box-shadow: var(--glow-cta);
-  transition: box-shadow 0.15s, opacity 0.15s;
+  font-weight: 500;
+  transition: background 0.15s;
 }
 .btn-edit:hover {
-  box-shadow: var(--glow-cta-hover);
-  opacity: 0.95;
+  background: var(--color-action-primary-hover);
   color: #fff;
 }
+/* Convert + Share: Bronze action-secondary — alternate paths */
 .btn-convert {
   margin-left: auto;
   padding: 0.3rem 0.75rem;
-  background: var(--color-bg-secondary);
-  color: var(--color-text);
-  border: 1px solid var(--color-border);
+  background: var(--color-action-secondary);
+  color: #fff;
+  border: none;
   border-radius: var(--radius-sm);
   cursor: pointer;
   font-size: 0.85rem;
+  transition: background 0.15s;
 }
-.btn-convert:hover {
-  background: var(--color-primary);
-  color: #fff;
-  border-color: var(--color-primary);
-}
+.btn-convert:hover { background: var(--color-action-secondary-hover); }
 .btn-convert:disabled {
   opacity: 0.6;
   cursor: default;
@@ -387,21 +385,21 @@ async function convertToTask() {
 
 .btn-share {
   padding: 0.3rem 0.75rem;
-  background: var(--color-bg-secondary);
-  border: 1px solid var(--color-border);
+  background: var(--color-action-secondary);
+  border: none;
   border-radius: var(--radius-sm);
-  color: var(--color-text-secondary);
+  color: #fff;
   cursor: pointer;
   font-size: 0.85rem;
   font-family: inherit;
-  transition: border-color 0.15s, color 0.15s;
+  transition: background 0.15s;
 }
-.btn-share:hover { border-color: var(--color-primary); color: var(--color-primary); }
+.btn-share:hover { background: var(--color-action-secondary-hover); }
 
 .note-title {
   font-family: "Fraunces", Georgia, serif;
   font-size: 2rem;
-  font-weight: 700;
+  font-weight: 500;
   line-height: 1.2;
   margin: 0.25rem 0 0.5rem;
   color: var(--color-text);
@@ -440,7 +438,7 @@ async function convertToTask() {
   align-items: center;
   gap: 0.4rem;
   font-size: 0.78rem;
-  font-weight: 700;
+  font-weight: 500;
   text-transform: uppercase;
   letter-spacing: 0.06em;
   color: var(--color-text-muted);
@@ -482,7 +480,7 @@ async function convertToTask() {
   font-size: 0.68rem;
   text-transform: uppercase;
   letter-spacing: 0.04em;
-  font-weight: 600;
+  font-weight: 500;
   padding: 0.1rem 0.45rem;
   border-radius: 999px;
   flex-shrink: 0;