feat(web): in-feed paste box + onboarding-aware connect confirmation

5b: connected users get a 'Summarize any video' URL input on the feed; submit
posts to /paste (HTMX) and swaps the resulting card / inline error into the feed.
7: the connect flash now sets expectations for the async onboarding burst —
'finding your subscriptions, your newest videos will appear below as summarized'.

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

internal/web/view.go

@@ -333,7 +333,7 @@ type flashView struct {
 // flashMessages maps each flash code to its banner. An unknown code renders no
 // banner (flashFor returns ok=false), so a forged cookie value is inert.
 var flashMessages = map[string]flashView{
-	flashConnected:     {"success", "YouTube account connected."},
+	flashConnected:     {"success", "YouTube account connected — finding your subscriptions. Your newest videos will appear below as they're summarized."},
 	flashConnectFailed: {"error", "Could not connect your YouTube account. Please try again."},
 	flashDisconnected:  {"success", "Account disconnected."},
 	flashDeleted:       {"success", "Your account and all its data were deleted."},