fix(site): fix sticky user message clipping and fade-in behavior (#23928)

The sticky user message in the chat timeline had two visual issues:

1. **Dead space during scroll** — the clipping calculation subtracted
48px prematurely (`fullHeight - scrolledPast - 48`), causing the message
to shrink before its content had actually left the viewport. Removed the
offset so clipping begins exactly when content scrolls out of view.

2. **Blur/gradient popping in abruptly** — the `--fade-opacity` variable
was a binary 0/1 toggle. Now it ramps 0→1 over the last 40px before
`MIN_HEIGHT`, so the blur and bottom gradient only appear when the
message is fully compressed.

Also added a longer (~25 line) user message to the `WithMessageHistory`
story to make the sticky behavior easier to test visually.

site/src/pages/AgentsPage/AgentChatPage.stories.tsx

@@ -351,7 +351,7 @@ export const WithMessageHistory: Story = {
 							},
 						],
 					},
-					// -- Turn 3: user follow-up --
+					// -- Turn 3: user follow-up (long message) --
 					{
 						id: 3,
 						chat_id: CHAT_ID,
@@ -360,7 +360,33 @@ export const WithMessageHistory: Story = {
 						content: [
 							{
 								type: "text",
-								text: "Can you show me the token validation code and a comparison of the old vs new approach?",
+								text: [
+									"Can you show me the token validation code and a comparison of the old vs new approach?",
+									"",
+									"I have a lot of context I want to share so you can give me the best possible answer.",
+									"The current token validation is scattered across multiple files and it is really hard",
+									"to follow the flow from HTTP request to database lookup to response. The middleware in",
+									"coderd/httpmw/apikey.go does way too much - it parses the token, validates the signature,",
+									"checks expiration, looks up the user, checks if the user is suspended, and then sets up",
+									"the context. That is at least 6 different responsibilities in a single middleware function.",
+									"",
+									"Here are the specific files I have been looking at:",
+									"- coderd/httpmw/apikey.go (main middleware, ~400 lines)",
+									"- coderd/httpmw/oauth2.go (OAuth2 token handling)",
+									"- coderd/httpmw/session.go (session cookie management)",
+									"- coderd/userauth.go (login/logout handlers)",
+									"- coderd/apikey.go (API key CRUD operations)",
+									"- enterprise/coderd/proxyhealth.go (proxy authentication)",
+									"",
+									"The problem is that ExtractAPIKeyMW is doing too many things at once:",
+									"1. Extracting the token from the request (cookie or header)",
+									"2. Splitting the token into key ID and secret",
+									"3. Looking up the API key in the database",
+									"4. Hashing the secret and comparing it",
+									"5. Checking if the key is expired",
+									"",
+									"Can you incorporate all of this into your comparison of the old vs new approach?",
+								].join("\n"),
 							},
 						],
 					},

site/src/pages/AgentsPage/components/ChatConversation/ConversationTimeline.tsx

@@ -602,6 +602,7 @@ const ChatMessageItem = memo<{
 										<div
 											className="pointer-events-none absolute inset-x-0 bottom-0 h-1/2 max-h-12"
 											style={{
+												opacity: "var(--fade-opacity, 0)",
 												background:
 													"linear-gradient(to top, hsl(var(--surface-secondary)), transparent)",
 											}}
@@ -793,15 +794,17 @@ const StickyUserMessage = memo<{
 					container.style.top = "0px";
 					return;
 				}
-				const visible = Math.max(fullHeight - scrolledPast - 48, MIN_HEIGHT);
+				const visible = Math.max(fullHeight - scrolledPast, MIN_HEIGHT);
 				container.style.setProperty("--clip-h", `${visible}px`);
-				// Only show the fade gradient once enough content is
-				// clipped to be visually meaningful.
-				container.style.setProperty(
-					"--fade-opacity",
-					visible < fullHeight - 8 ? "1" : "0",
+				// Only show the blur and gradient once the message
+				// is near its minimum compressed height. Ramp over
+				// the last 40px before MIN_HEIGHT so it doesn't pop.
+				const FADE_RANGE = 40;
+				const fade = Math.max(
+					0,
+					Math.min((MIN_HEIGHT + FADE_RANGE - visible) / FADE_RANGE, 1),
 				);
-
+				container.style.setProperty("--fade-opacity", String(fade));
 				// Push-up effect: when the next user message's sentinel
 				// approaches the bottom of this sticky container, shift
 				// this container upward so it slides out of view — the
@@ -960,6 +963,7 @@ const StickyUserMessage = memo<{
 							<div
 								className="absolute inset-0 backdrop-blur-[1px] bg-surface-primary/15"
 								style={{
+									opacity: "var(--fade-opacity, 0)",
 									maxHeight: "calc(var(--clip-h, 100%) + 48px)",
 									willChange: "max-height, mask-image",
 									maskImage: