feat(go): chat UI overhaul — copy, collapse, html blocks, content sidebar + chips

[baryhuang]

Native chat-surface UI overhaul for the iOS/macOS Go app. Six improvements ship together.

1. Copy button on agent messages

Small Copy / Copied button under each agent message that writes the raw markdown to the system pasteboard so output roundtrips to other markdown-aware tools without losing structure. Mirrors the React UI affordance.

2. Collapse thinking steps to last 2

Long thinking / tool-call chains used to overwhelm the chat. Now only the most recent two steps render by default; earlier ones collapse behind a tappable N earlier steps header that unrolls the full list. Rolling window — while the agent keeps producing steps, the last-2 view tracks the latest.

3. Render fenced ```html blocks inline

New MarkdownSegment.htmlBlock variant alongside .code / .table. The segmenter routes ```html fences to it; the bubble renderer mounts them in a sandboxed WKWebView.

• JavaScript disabled at the WKPreferences level — default is safe preview, not arbitrary script execution.
• Document scaffold sets color-scheme so embedded inputs/forms match the surrounding chat theme.
• Reported height capped at 400 pt with internal scroll so a long artifact doesn't blow out the chat scroll position.
• xml, htm, code, etc. still render as code blocks — only the explicit html tag opts into the live preview.

4. Content sidebar listing workspace files

Right-hand Content panel toggled from a sidebar.right icon in the chat header (macOS) / top-bar toolbar (iOS). Lists every file in the workspace, most-recent first.

• Default off. Side-by-side on macOS / iPad regular hsize; slides up as a sheet on iPhone compact since the sidebar would otherwise crush the chat.
• Resizable. Drag the handle between chat and sidebar to widen; resize cursor on macOS via NSCursor.resizeLeftRight. Width clamps between 280 pt (single column) and 560 pt (two columns).
• File list is a LazyVGrid with .adaptive(minimum: 240), so the layout switches to 2 columns automatically once the user drags the sidebar wide enough.
• Image files render a thumbnail via a small AuthorizedAsyncImage loader (since AsyncImage can't carry headers and /v1/files/{id} requires X-Workspace-Token).
• Non-image kinds get a tinted badge + SF Symbol so the type is legible at a glance.

Backend unchanged: GET /v1/files?network=<id>&status=active&limit=100.

5. Workspace-file chips + sidebar file detail

When an agent posts a link to a workspace file (/v1/files/<id>) on its own line, it renders as a tappable pill instead of an inline anchor. Tapping the chip opens the Content sidebar focused on that file. Tapping a file in the sidebar list opens the same detail panel. A back chevron returns to the file list.

• ContentSidebarController is the shared state owner (isPresented + selectedFileId). Surfaced through @Environment so chips drive navigation without prop-drilling.
• MarkdownSegments.fileChip(fileId, label) segment; a post-pass over prose segments splits out lines matching [label](https://.../v1/files/UUID) or bare URLs. Whole-line matches only — chips don't stamp into the middle of a sentence.
• FileDetailView renders content by kind: image via AuthorizedAsyncImage (.fit), text/code via UTF-8 decode in a monospaced scroll view (capped at 512 KB), HTML via the shared sandboxed WebView. PDF/audio/video/archive get a stub card.
• WebView and AuthorizedAsyncImage extracted into Helpers/ so both HTMLBlockView (in-chat) and FileDetailView (sidebar) can reuse them.
• New GET /v1/files/{id}/info wrapper for authoritative metadata when we only have an id from a chip. Returns metadata without a status field (only the list endpoint does), so the Swift model accepts status as optional and decodes both endpoint shapes.

6. Chat history scroll-to-bottom + loading state

History load used to leave the chat blank or stuck part-way down — users had to manually scroll up before the latest messages painted. The empty-state guard also treated "load in flight" identically to "no messages here", so threads showed the "say hi" copy during the network round-trip.

Root cause was LazyVStack racing ScrollViewReader.scrollTo("bottom-anchor"): the 1pt anchor sat at the bottom of an essentially-empty content height because rows above hadn't been instantiated yet.

• Added loadingHistory to ChannelMessages; loadHistory sets it before the await so the chat view can show a spinner.
• Replaced the racing .onAppear { scrollTo } with .defaultScrollAnchor(.bottom) — iOS 17 / macOS 14 primitive that anchors the bottom of content to the viewport across content-size changes. Also stops "load older" prepends from jumping visible content.
• .id(session.sessionId) on the ScrollView so thread switches re-apply the anchor cleanly.

Test plan

• iOS Debug clean
• macOS Debug clean
• macOS Release DMG built — packages/go/dist/OpenAgents Go-0.2.4-arm64.dmg
• Manual: open a long agent message → tap Copy → paste into Bear/Notion → markdown preserved.
• Manual: trigger a long thinking run (>5 steps) → only last 2 visible by default → tap header → full history expands.
• Manual: agent posts a ```html fence → renders as styled HTML; JS does NOT execute.
• Manual: toggle Content sidebar → file list appears, sorted recent first; image files show thumbnails; drag the left edge → sidebar grows; at ~560pt the grid becomes 2 columns.
• Manual: agent posts a markdown link to a workspace file → renders as a pill → tap → sidebar opens to detail view; back chevron returns to list.
• Manual: switch threads → chat lands at the bottom of the new thread on first paint, not stuck at top; spinner shows during load instead of "say hi" copy.
• Manual on iPhone: toggle Content from toolbar → sheet slides up rather than crushing the chat.

[vercel]

@baryhuang is attempting to deploy a commit to the Raphael's projects Team on Vercel.

A member of the Team first needs to authorize it.

[zomux]

Clean, well-structured UI overhaul. WKWebView sandboxing is correct (JS disabled + baseURL nil). No bugs or security concerns. Minor nits (non-blocking): ISO8601DateFormatter could be shared/static, and byteString/relativeTime helpers are duplicated between FileCard and FileDetailView.