feat(design): surface phase PR 3 — Chat surface polish

Per the surface-phase spec for the Chat cluster (ChatView, ChatPanel,
ChatMessage, ChatInputBar, ToolCallCard):

Long-form line-height
- ChatMessage: assistant-bubble .message-content jumps from 1.55 to
  1.7 — chat is a reading surface, not a snippet stream. User
  bubbles stay tighter.
- JournalView: dropped its :deep(.role-assistant .message-content)
  override; ChatMessage handles it now and Journal inherits.

ToolCallCard borders
- Removed the outer 1px border. ToolCallCard always renders inside
  an assistant bubble; the bubble already contains it. Background
  tint differentiates without re-bordering. Per the
  structural-not-decorative rule.
- Error state preserved as a 3px left-edge accent in --color-danger,
  mirroring the assistant bubble's own left-edge pattern.

Button reclassification per Hybrid rule
- ChatView .bulk-link "All"/"None": accent text → --color-text-secondary
  (these are tertiary list-control affordances, not brand moments)
- ChatView .bulk-delete-btn: --color-danger (Error terracotta) →
  --color-action-destructive (Oxblood) per Hybrid; paired with a
  Trash2 icon since destructive should always be reinforced by an
  icon, not just color
- ChatView .btn-delete-conv hover: same Error → Oxblood swap
- .btn-new-conv stays accent (brand moment, correct already)
- .btn-send stays accent gradient (primary brand moment, foundation)

Two-weights-only
- Snapped every font-weight: 600/700 to 500 across ChatView,
  ChatPanel, ChatMessage, ToolCallCard per the doc rule.

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

frontend/src/components/ChatMessage.vue

@@ -155,7 +155,7 @@ const timingParts = computed((): string[] => {
 }
 .role-label {
   font-size: 0.75rem;
-  font-weight: 600;
+  font-weight: 500;
   text-transform: uppercase;
   letter-spacing: 0.03em;
 }
@@ -182,7 +182,7 @@ const timingParts = computed((): string[] => {
 .thinking-summary {
   padding: 0.25rem 0.5rem;
   font-size: 0.72rem;
-  font-weight: 600;
+  font-weight: 500;
   text-transform: uppercase;
   letter-spacing: 0.04em;
   color: var(--color-text-muted);
@@ -215,6 +215,12 @@ details[open] .thinking-summary::before {
   overflow-y: auto;
   background: var(--color-bg-secondary);
 }
+/* Long-form line-height (1.7) on assistant bubbles per the design system —
+   chat is a reading surface, not a snippet stream. User bubbles stay tighter. */
+.role-assistant .message-content {
+  line-height: 1.7;
+}
+
 .message-content {
   font-size: 0.95rem;
   line-height: 1.55;