CL-6376/CL-6377: timeline polish + single Connect action

[TheGreatAxios]

Summary

CL-6376 (chat timeline polish, packages/chat-ui):

• Failed-turn notice redesigned as a quiet inline system row aligned with the message column (.chat-turn-failed) instead of PrFailedTurnStrip's bordered banner — muted danger text, a small ghost Retry, "What happened" as an inline disclosure. Auto-retry-once semantics and consumer copy unchanged.
• The three-dot typing indicator now also clears when a turn fails via postUndeliveredNotice (a chat.message with a turnFailed part) — previously that path emitted no chat.agent events, so the pulse stayed stranded until a 120s backstop.
• The pin-toggle glyph no longer mounts for an unpinned message just because a host wires pinActions — it only renders once there's something to show (a reaction, or an already-pinned message needing a visible unpin). Pinning stays reachable via the ellipsis/context menu's "Pin message" entry.
• Fixed the broken greeting interpolation: "Welcome in alice" → "Welcome in, alice."
• The hover action toolbar now anchors to each message's own content wrapper rather than the whole .chat-message-group (which could include a leading day divider) — it no longer floats up into the divider's space on the first message of a new day.

CL-6377 (connect flow, packages/connections + settings-ui + plugins-ui + chat-ui):

• Collapsed the "test key, then save" two-call flow into a single Connect action everywhere (Connections settings dialog, workbench plugins panel, plugins-ui connect card). POST /:connectorId/complete already proves the key against the connector's own probe before storing it, so the separate client-driven /credential/test round-trip was dead weight.
• Removed the POST /:connectorId/credential/test route and its client helpers (testConnectorCredential) entirely, server and client — no fallback path left behind.
• Button copy is "Connect"/"Connecting…" everywhere; a rejected key surfaces inline from the same connect call's 422.

CL-6378 (tool_use rendering as raw JSON): investigated but not started — didn't fit this box. tool-trace parts already render through react-ui's ToolBlock lifecycle component, not raw text, so the leak is upstream of the renderer (how a turn's final text content is assembled in chat-orchestrator.ts / @corbits/folded-runs' connectorReplyContent, or a different message path) and needs its own investigation pass.

Test plan

• bun test + bunx tsc --noEmit green in packages/chat-ui, packages/chat, packages/settings-ui, packages/connections, packages/plugins-ui
• bunx eslint src clean in touched packages (pre-existing warnings elsewhere untouched)
• New/updated tests: typing-pulse clears on turnFailed (streaming-reply.test.ts), failed-turn row selectors, pin-toggle-hidden-until-pinned + reachable via context menu, connect-dialog single round-trip + inline 422 error