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/hooks/useSessionStream.ts

@@ -40,6 +40,7 @@ function applyFrame(state: State, frame: WsFrame): State {
         tokens_used: null,
         ctx_used: null,
         ctx_max: null,
+        model: null,
         started_at: null,
         finished_at: null,
         created_at: new Date().toISOString(),
@@ -105,6 +106,7 @@ function applyFrame(state: State, frame: WsFrame): State {
         tokens_used: null,
         ctx_used: null,
         ctx_max: null,
+        model: null,
         started_at: null,
         finished_at: null,
         created_at: new Date().toISOString(),
@@ -123,6 +125,7 @@ function applyFrame(state: State, frame: WsFrame): State {
               ...(frame.ctx_max !== undefined ? { ctx_max: frame.ctx_max } : {}),
               ...(frame.started_at !== undefined ? { started_at: frame.started_at } : {}),
               ...(frame.finished_at !== undefined ? { finished_at: frame.finished_at } : {}),
+              ...(frame.model !== undefined ? { model: frame.model } : {}),
               // v1.8.2: cap-hit sentinels (and future stamped metadata) ride
               // in on this terminal frame so the reducer can attach it
               // without waiting for a refetch.