Add Windows Hub UI for gateway session maintenance settings

Problem

Windows Hub exposes the gateway's sessions list and a generic configuration editor, but it does not provide a dedicated UI for managing session.maintenance settings such as mode, pruneAfter, maxEntries, resetArchiveRetention, maxDiskBytes, and highWaterBytes.

Operators using Windows Hub should not need to edit raw gateway configuration to control session retention and disk usage.

Proposed UI

Add a Session maintenance section to the existing Gateway Configuration page that:

• loads the current session.maintenance values from the connected gateway
• exposes the supported settings with defaults and concise explanations
• saves the full updated config through config.patch with optimistic concurrency
• preserves unrelated gateway configuration
• clearly handles disconnected, read-only, loading, validation, conflict, and save-result states

Acceptance criteria

• Windows Hub can view and update all supported session.maintenance settings.
• Omitted values display gateway defaults without unnecessarily materializing them until the user saves.
• Numeric and duration/disk-size inputs are validated before saving.
• Changes participate in the Configuration page's existing Save/Discard flow, preserve unrelated config, and refresh the gateway snapshot.
• Focused model/UI contract tests cover parsing, validation, patch construction, and the Sessions page surface.
• Current-head UI proof is attached, or an explicit Windows-host blocker is documented.

Upstream reference: docs/reference/session-management-compaction.md in openclaw/openclaw.

Start with the existing Gateway Configuration page and its Save/Discard flow, then read docs/reference/session-management-compaction.md for the session.maintenance settings. Trace the config.patch path and existing model/UI tests before adding coverage for parsing, validation, patch construction, and the Sessions page surface. Done means all listed settings and states work without altering unrelated configuration, with current-head UI proof or a documented Windows-host blocker.