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/ChatPanel.vue

@@ -580,7 +580,7 @@ defineExpose({ focus, prefill, send, contextCount, sidebarOpen, toggleContextSid
   padding: 0.15rem 0.1rem;
   font-family: inherit;
   font-size: 0.65rem;
-  font-weight: 700;
+  font-weight: 500;
   text-transform: uppercase;
   letter-spacing: 0.05em;
   color: var(--color-text-muted);
@@ -598,7 +598,7 @@ defineExpose({ focus, prefill, send, contextCount, sidebarOpen, toggleContextSid
 .context-sidebar-header .section-count {
   margin-left: auto;
   color: var(--color-text-muted);
-  font-weight: 600;
+  font-weight: 500;
   background: var(--color-bg);
   padding: 0.05rem 0.3rem;
   border-radius: 8px;
@@ -622,7 +622,7 @@ defineExpose({ focus, prefill, send, contextCount, sidebarOpen, toggleContextSid
 }
 .auto-pill {
   font-size: 0.55rem;
-  font-weight: 700;
+  font-weight: 500;
   letter-spacing: 0.05em;
   padding: 0.05rem 0.3rem;
   border-radius: 4px;
@@ -686,7 +686,7 @@ defineExpose({ focus, prefill, send, contextCount, sidebarOpen, toggleContextSid
   text-transform: uppercase;
   letter-spacing: 0.05em;
   color: var(--color-text-muted);
-  font-weight: 700;
+  font-weight: 500;
   margin-bottom: 0.2rem;
 }
 .queued-clear-row {
@@ -734,7 +734,7 @@ defineExpose({ focus, prefill, send, contextCount, sidebarOpen, toggleContextSid
 
 .empty-section-label {
   font-size: 0.7rem;
-  font-weight: 600;
+  font-weight: 500;
   text-transform: uppercase;
   letter-spacing: 0.08em;
   color: var(--color-text-muted);
@@ -871,7 +871,7 @@ defineExpose({ focus, prefill, send, contextCount, sidebarOpen, toggleContextSid
 .btn-open-chat.prominent {
   font-size: 0.9rem;
   color: var(--color-primary);
-  font-weight: 600;
+  font-weight: 500;
 }
 .btn-clear-response {
   background: none;
@@ -918,7 +918,7 @@ defineExpose({ focus, prefill, send, contextCount, sidebarOpen, toggleContextSid
 :deep(.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);