feat(web): add copy button to user messages (#349)

* feat(web): add copy button to user messages

Add a small copy button to user message bubbles for easy text copying,
especially useful on mobile where selecting text is difficult.

- Mobile: button always visible (opacity-60)
- Desktop: button appears on hover
- Uses existing useCopyToClipboard hook with haptic feedback
- Conditionally rendered to avoid empty container spacing

via [HAPI](https://hapi.run)

Co-Authored-By: HAPI <noreply@hapi.run>

* fix(web): use valid CSS property in copy button transition

transition-[opacity,colors] is invalid because 'colors' is not a CSS
property (only Tailwind's utility class 'transition-colors' expands it).
Use 'background-color' instead so the hover background transition
actually works.

via [HAPI](https://hapi.run)

Co-Authored-By: HAPI <noreply@hapi.run>

---------

Co-authored-by: HAPI <noreply@hapi.run>

web/src/components/AssistantChat/messages/UserMessage.tsx

@@ -5,9 +5,12 @@ import type { HappyChatMessageMetadata } from '@/lib/assistant-runtime'
 import { MessageStatusIndicator } from '@/components/AssistantChat/messages/MessageStatusIndicator'
 import { MessageAttachments } from '@/components/AssistantChat/messages/MessageAttachments'
 import { CliOutputBlock } from '@/components/CliOutputBlock'
+import { CopyIcon, CheckIcon } from '@/components/icons'
+import { useCopyToClipboard } from '@/hooks/useCopyToClipboard'
 
 export function HappyUserMessage() {
     const ctx = useHappyChatContext()
+    const { copied, copy } = useCopyToClipboard()
     const role = useAssistantState(({ message }) => message.role)
     const text = useAssistantState(({ message }) => {
         if (message.role !== 'user') return ''
@@ -58,17 +61,29 @@ export function HappyUserMessage() {
     const hasAttachments = attachments && attachments.length > 0
 
     return (
-        <MessagePrimitive.Root className={userBubbleClass}>
+        <MessagePrimitive.Root className={`${userBubbleClass} group/msg`}>
             <div className="flex items-end gap-2">
                 <div className="flex-1 min-w-0">
                     {hasText && <LazyRainbowText text={text} />}
                     {hasAttachments && <MessageAttachments attachments={attachments} />}
                 </div>
-                {status ? (
-                    <div className="shrink-0 self-end pb-0.5">
-                        <MessageStatusIndicator status={status} onRetry={onRetry} />
+                {(hasText || status) && (
+                    <div className="shrink-0 self-end pb-0.5 flex items-center gap-1">
+                        {hasText && (
+                            <button
+                                type="button"
+                                title="Copy"
+                                className="opacity-60 sm:opacity-0 sm:group-hover/msg:opacity-100 transition-[opacity,background-color] p-0.5 rounded hover:bg-[var(--app-subtle-bg)]"
+                                onClick={() => copy(text)}
+                            >
+                                {copied
+                                    ? <CheckIcon className="h-3.5 w-3.5 text-green-500" />
+                                    : <CopyIcon className="h-3.5 w-3.5 text-[var(--app-hint)]" />}
+                            </button>
+                        )}
+                        {status && <MessageStatusIndicator status={status} onRetry={onRetry} />}
                     </div>
-                ) : null}
+                )}
             </div>
         </MessagePrimitive.Root>
     )