UI: flip bubble hierarchy — responses elevated, prompts recessed

Design direction: "Illuminated Transcript" — the AI response is the
primary artifact; user prompts are subordinate margin notes.

ChatMessage.vue:
- User bubble: ghosted background (rgba 3-4% opacity), muted text,
  thin near-invisible border. De-emphasized, recessed.
- Assistant bubble: left indigo accent border (2px), elevated shadow
  via --color-bubble-asst-shadow. Commands attention.
- Assistant role label: indigo (var(--color-primary)); user label muted.
- Removed old white-override content rules (no longer on dark bg).
- Thinking block: full border → left-only indigo accent bar (35% opacity),
  removed inner top border.

ChatView.vue:
- Streaming bubble: same left accent + shadow as assistant bubbles.
- Streaming role label: indigo to match ChatMessage.
- Thinking block in streaming: same treatment as ChatMessage.

theme.css:
- Added --color-bubble-user-bg/border/text for light + dark.
- Added --color-bubble-asst-shadow for light + dark.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/assets/theme.css

@@ -38,6 +38,10 @@
   --color-input-bar-text: #1a1a1a;
   --color-input-bar-placeholder: rgba(0, 0, 0, 0.4);
   --color-overlay: rgba(0, 0, 0, 0.45);
+  --color-bubble-user-bg: rgba(0, 0, 0, 0.04);
+  --color-bubble-user-border: rgba(0, 0, 0, 0.08);
+  --color-bubble-user-text: #555566;
+  --color-bubble-asst-shadow: 0 2px 16px rgba(99, 102, 241, 0.10), 0 1px 4px rgba(0, 0, 0, 0.06);
   --radius-sm: 6px;
   --radius-md: 12px;
   --radius-lg: 18px;
@@ -85,6 +89,10 @@
   --color-input-bar-text: #e4e4f0;
   --color-input-bar-placeholder: rgba(228, 228, 240, 0.35);
   --color-overlay: rgba(0, 0, 0, 0.65);
+  --color-bubble-user-bg: rgba(255, 255, 255, 0.03);
+  --color-bubble-user-border: rgba(255, 255, 255, 0.07);
+  --color-bubble-user-text: #5a5a72;
+  --color-bubble-asst-shadow: 0 4px 28px rgba(99, 102, 241, 0.14), 0 2px 8px rgba(0, 0, 0, 0.4);
 }
 
 *,