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/assets/editor-shared.css

@@ -50,34 +50,42 @@
   border-color: var(--color-primary);
   color: var(--color-primary);
 }
+/* Save: Moss action-primary per the Hybrid rule. Saving is "operating
+   the software" — not a brand moment. Accent gradient is reserved for
+   Send / empty-state CTAs. */
 .btn-save {
   padding: 0.45rem 1.1rem;
-  background: linear-gradient(135deg, #5B4A8A, #3F3560);
+  background: var(--color-action-primary);
   color: #fff;
   border: none;
   border-radius: var(--radius-sm);
   cursor: pointer;
-  font-weight: 600;
+  font-weight: 500;
   font-size: 0.875rem;
-  box-shadow: 0 2px 8px rgba(91, 74, 138, 0.28);
-  transition: box-shadow 0.15s, opacity 0.15s;
+  transition: background 0.15s, opacity 0.15s;
 }
 .btn-save:hover:not(:disabled) {
-  box-shadow: 0 4px 14px rgba(91, 74, 138, 0.42);
-  opacity: 0.95;
+  background: var(--color-action-primary-hover);
 }
 .btn-save:disabled {
   opacity: 0.55;
   cursor: default;
 }
+/* Delete: Oxblood action-destructive per Hybrid rule. Should be paired
+   with a Trash icon at the call site to reinforce intent. */
 .btn-delete {
   padding: 0.45rem 1rem;
-  background: var(--color-danger);
+  background: var(--color-action-destructive);
   color: #fff;
   border: none;
   border-radius: var(--radius-sm);
   cursor: pointer;
+  display: inline-flex;
+  align-items: center;
+  gap: 0.35rem;
+  font-weight: 500;
 }
+.btn-delete:hover { background: var(--color-action-destructive-hover); }
 .btn-assist-toggle {
   margin-left: auto;
   padding: 0.4rem 0.9rem;
@@ -99,7 +107,7 @@
   border-bottom: 1.5px solid var(--color-border);
   border-radius: 0;
   font-size: 1.5rem;
-  font-weight: 700;
+  font-weight: 500;
   font-family: "Fraunces", Georgia, serif;
   background: transparent;
   color: var(--color-text);
@@ -225,7 +233,7 @@
 .assist-panel-title {
   flex: 1;
   font-size: 0.8rem;
-  font-weight: 700;
+  font-weight: 500;
   color: var(--color-text-secondary);
   text-transform: uppercase;
   letter-spacing: 0.05em;
@@ -269,7 +277,7 @@
 /* Section list */
 .assist-sections-label {
   font-size: 0.72rem;
-  font-weight: 700;
+  font-weight: 500;
   text-transform: uppercase;
   letter-spacing: 0.04em;
   color: var(--color-text-muted);
@@ -411,7 +419,7 @@
   align-items: center;
   justify-content: space-between;
   font-size: 0.8rem;
-  font-weight: 600;
+  font-weight: 500;
   color: var(--color-text-secondary);
 }
 .btn-toggle-view {
@@ -454,7 +462,7 @@
   width: 1rem;
   text-align: center;
   user-select: none;
-  font-weight: 700;
+  font-weight: 500;
 }
 .diff-text {
   flex: 1;
@@ -562,7 +570,7 @@
   border: none;
   border-bottom: 1px solid var(--color-border);
   font-size: 0.85rem;
-  font-weight: 600;
+  font-weight: 500;
   color: var(--color-text-secondary);
   cursor: pointer;
   text-align: left;
@@ -581,7 +589,7 @@
 }
 .sb-label {
   font-size: 0.78rem;
-  font-weight: 600;
+  font-weight: 500;
   color: var(--color-text-secondary);
   text-transform: uppercase;
   letter-spacing: 0.04em;