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/prose.css

@@ -1,5 +1,8 @@
 .prose {
-  line-height: 1.6;
+  /* Long-form reading line-height per the design system —
+     reading surfaces (notes, tasks, journal, assistant bubbles) want 1.7
+     so prose breathes like a book, not a UI snippet. */
+  line-height: 1.7;
 }
 
 .prose h1 {