feat: BooCode 2.0 UI — Ember theme, brand banner, coder tabs, model-attribution chips

- Ember theme (Obsidian charcoal + #ff7a18 orange), now DEFAULT_THEME_ID; server theme_id whitelist gains 'ember'
- Brand banner: transparent Westie mascot + >_BooCode wordmark, big/edge-to-edge (flood-filled to transparency + cropped)
- Coder panes are multi-tab: + opens a BooCode tab, split opens a pane (shared ChatTabBar via tabKind + createCoderTab; closeOtherTabs/tab-numbering extended to coder)
- Model-attribution: new messages.model column stamped at finalizeCompletion (BooChat/native coder) + dispatcher assistant-row creation (external coder); surfaced via view + wire types + live frame; rendered as a subtle shortened-name chip (shortenModelName)
- Composer Web toggle moved into a boxed focus-ringed input; glowing accent dot on tool rows
- Claude SDK follow-ups (1M context, follow-up-message fix, collapsed thinking/tool chips) + CLAUDE_SDK_BACKEND=1

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/web/src/lib/theme.ts

@@ -24,7 +24,8 @@ export type ThemeId =
   | 'ivory'
   | 'chalk'
   | 'cobalt'
-  | 'midnight-sapphire';
+  | 'midnight-sapphire'
+  | 'ember';
 
 export type ThemeMode = 'dark' | 'light' | 'system';
 
@@ -74,9 +75,13 @@ export const THEMES: readonly ThemeMeta[] = [
     anchors: ['#020817', '#061434', '#0c2244', '#3060a0', '#0047ab'] },
   { id: 'midnight-sapphire', name: 'Midnight Sapphire', family: 'Blue', supportsDark: true, supportsLight: true,
     anchors: ['#02050e', '#060c1f', '#0e1a36', '#4a6088', '#1e3a8a'] },
+  { id: 'ember', name: 'BooCode Ember', family: 'Amber', supportsDark: true, supportsLight: true,
+    anchors: ['#0c0c0e', '#15151a', '#1f1f23', '#6b6b75', '#ff7a18'] },
 ] as const;
 
-export const DEFAULT_THEME_ID: ThemeId = 'obsidian';
+// BooCode 2.0: orange-on-black "BooCode Ember" is the out-of-the-box signature
+// (was 'obsidian' / purple). Also the dark fallback for the light-only themes.
+export const DEFAULT_THEME_ID: ThemeId = 'ember';
 export const DEFAULT_THEME_MODE: ThemeMode = 'dark';
 export const STORAGE_KEY = 'boocode.theme';