refactor(web): dock chat inline below the summary, integrated view (ADR-027)

The chat was a separate page — opening it left the summary behind, the very
context you're asking about. Now the chat docks open IN PLACE below the summary:
"Dig deeper" reveals the chat section (HTMX, outerHTML over the closed dock) so
the summary stays on screen above it; no navigation. The no-JS fallback renders
the full summary AND the open chat on one page (the same integrated view), so
progressive enhancement holds.

Extracts a shared summaryBody templ so the detail page and the chat page render
one identical summary, not two divergent ones. GET /v/{id}/chat returns just the
open chat section as a fragment for the inline reveal, or the full summary+chat
page for a no-JS navigation; POST swaps the panel inline or re-renders the whole
page. Tests assert summary+chat coexist on the page and the reveal is a fragment.

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

internal/web/chat.go

@@ -64,13 +64,21 @@ func (a *App) handleChat(w http.ResponseWriter, r *http.Request) {
 	if !ok {
 		return
 	}
-	a.render(w, r, ChatPage(chatView{
+	view := chatView{
 		VideoID:   row.VideoID,
 		Title:     displayTitle(*row),
 		Available: hasText,
 		Models:    a.Chat.Models(),
 		Selected:  a.Chat.DefaultModel(row.AIModel),
-	}))
+	}
+	// HTMX (the in-place reveal from the summary) gets just the open chat section,
+	// swapped over the closed dock so the summary above it stays put. A no-JS
+	// navigation gets the full page: the whole summary plus the open chat.
+	if isHTMX(r) {
+		a.render(w, r, chatSection(view))
+		return
+	}
+	a.render(w, r, ChatPage(*row, view))
 }
 
 // handleChatMessage answers one question against the stored transcript (POST).
@@ -129,7 +137,7 @@ func (a *App) handleChatMessage(w http.ResponseWriter, r *http.Request) {
 			view.Truncated = reply.Truncated
 		}
 	}
-	a.renderChat(w, r, view)
+	a.renderChatTurn(w, r, *row, view)
 }
 
 // loadOwnedSummary fetches the summary for the path's video scoped to userID, or
@@ -166,14 +174,15 @@ func (a *App) readTranscript(w http.ResponseWriter, r *http.Request, row store.S
 	return t.Content, true, true
 }
 
-// renderChat returns the chat panel fragment for an HTMX request, or the full
-// chat page otherwise (no-JS POST re-renders the whole page).
-func (a *App) renderChat(w http.ResponseWriter, r *http.Request, v chatView) {
+// renderChatTurn returns the chat panel fragment for an HTMX answer (swapped in
+// place within the open dock), or the full chat page otherwise — the no-JS POST
+// re-renders the whole summary + open chat with the new turn.
+func (a *App) renderChatTurn(w http.ResponseWriter, r *http.Request, row store.SummaryRow, v chatView) {
 	if isHTMX(r) {
 		a.render(w, r, chatPanel(v))
 		return
 	}
-	a.render(w, r, ChatPage(v))
+	a.render(w, r, ChatPage(row, v))
 }
 
 // resolveModel keeps the posted model only when it is an offered option; anything