fix(ui): surface compaction checkpoints in chat history

Fixes #76415.

- Explains compacted history boundaries in WebChat.
- Adds an Open checkpoints action for pre-compaction recovery.
- Updates WebChat docs and changelog with Thanks @BunsDev.
- Validated targeted UI tests, formatting/diff checks, Testbox changed gate, and exact-head CI.

Security: UI/docs/tests/styles-only change that reuses existing checkpoint APIs; no new dependencies, filesystem reads, workflow changes, or secret handling.

docs/web/webchat.md

@@ -25,6 +25,7 @@ Status: the macOS/iOS SwiftUI chat UI talks directly to the Gateway WebSocket.
 - The UI connects to the Gateway WebSocket and uses `chat.history`, `chat.send`, and `chat.inject`.
 - `chat.history` is bounded for stability: Gateway may truncate long text fields, omit heavy metadata, and replace oversized entries with `[chat.history omitted: message too large]`.
 - `chat.history` follows the active transcript branch for modern append-only session files, so abandoned rewrite branches and superseded prompt copies are not rendered in WebChat.
+- Compaction entries render as an explicit compacted-history divider. The divider explains that earlier turns are preserved in a checkpoint and links to the Sessions checkpoint controls, where operators can branch or restore the pre-compaction view when their permissions allow it.
 - Control UI remembers the backing Gateway `sessionId` returned by `chat.history` and includes it on follow-up `chat.send` calls, so reconnects and page refreshes continue the same stored conversation unless the user starts or resets a session.
 - Control UI coalesces duplicate in-flight submits for the same session, message, and attachments before generating a new `chat.send` run id; the Gateway still dedupes repeated requests that reuse the same idempotency key.
 - `chat.history` is also display-normalized: runtime-only OpenClaw context,