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.