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>

test/acceptance/scenario_coverage_test.go

@@ -65,6 +65,7 @@ var scenarioCoverage = map[string]string{
 
 	// chat_transcript.feature (ADR-027)
 	"A summary view offers a deeper-dive into the video":         "TestChatEntryAffordanceOnSummaryView",
+	"The summary and the chat are on one page":                   "TestChatIntegratedWithSummaryOnSamePage",
 	"Ask a question answered from the stored transcript":         "TestChatAnswersFromStoredTranscriptWithoutAnyFetch",
 	"Chat never fetches captions or reaches YouTube":             "TestChatAnswersFromStoredTranscriptWithoutAnyFetch",
 	"A video with no stored transcript offers no chat":           "TestChatUnavailableWhenNoStoredTranscript",