feat: OpenClaw session import with sorted history

Backend:
- New OpenClawEndpoint with GET /api/openclaw/sessions and POST /api/openclaw/import
- Lists all OpenClaw sessions with preview messages, model, summary
- Imports user/assistant text messages (skips tool-use/tool-result)
- Cleans user messages (removes webchat prefixes, cron tags, conversation info blocks)
- Re-import support: deletes previous openclaw-import chains before importing
- New DatabaseService methods: CreateChainForImport, CreateMessageForImport, DeleteOpenClawImport
- Added SortKey to MessageRecord (computed: openclaw-import gets id-10B, else id)

History sorting:
- OpenClaw-imported messages always appear before native messages
- Sort by CASE WHEN tag='openclaw-import' THEN id-10000000000 ELSE id END
- Cursor-based paging updated from beforeId/afterId to beforeSortKey/afterSortKey
- All history queries (GetLatestActive, Before, After, CountBefore) updated

Frontend:
- Dashboard: new 'OpenClaw Import' menu item in session card dropdown
- Modal: shows OpenClaw sessions with metadata, summary, preview messages
- Imported messages display blue 'OC' badge in chat
- API functions: apiGetOpenclawSessions, apiImportOpenclawSession
- History paging updated to use sortKey cursors

wwwroot/css/chat.css

@@ -1208,3 +1208,131 @@
   border-color: var(--f1-border);
   color: var(--muted);
 }
+
+/* ── OpenClaw Import Badge ── */
+
+.openclaw-badge {
+  display: inline-flex;
+  align-items: center;
+  gap: 4px;
+  font-size: 0.7em;
+  padding: 2px 6px;
+  border-radius: var(--border-radius);
+  background: rgb(from var(--primary) r g b / 0.15);
+  color: var(--primary);
+  font-weight: 600;
+  margin-bottom: 6px;
+  letter-spacing: 0.02em;
+}
+
+.openclaw-badge i {
+  font-size: 0.9em;
+}
+
+/* ── OpenClaw Import Modal ── */
+
+.openclaw-import-modal {
+  max-width: 700px;
+}
+
+.openclaw-hint {
+  font-size: var(--small-font-size);
+  color: var(--muted);
+  margin-bottom: var(--box-spacing);
+}
+
+.openclaw-session-list {
+  display: flex;
+  flex-direction: column;
+  gap: var(--box-spacing);
+}
+
+.openclaw-session-card {
+  border: var(--border-width) solid var(--f2-border);
+  border-radius: var(--border-radius);
+  padding: var(--box-padding);
+  cursor: pointer;
+  transition: border-color 0.15s, background 0.15s;
+}
+
+.openclaw-session-card:hover {
+  border-color: var(--primary);
+  background: rgb(from var(--primary) r g b / 0.05);
+}
+
+.openclaw-card-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: var(--box-spacing);
+  margin-bottom: 6px;
+  flex-wrap: wrap;
+}
+
+.openclaw-session-id {
+  font-family: var(--mono-font);
+  font-size: var(--small-font-size);
+  color: var(--primary);
+  font-weight: 600;
+}
+
+.openclaw-card-meta {
+  display: flex;
+  gap: 12px;
+  flex-wrap: wrap;
+}
+
+.openclaw-meta-item {
+  font-size: var(--small-font-size);
+  color: var(--muted);
+  white-space: nowrap;
+}
+
+.openclaw-meta-item i {
+  margin-right: 3px;
+}
+
+.openclaw-summary {
+  font-size: var(--small-font-size);
+  color: var(--text-color);
+  margin-bottom: 8px;
+  line-height: 1.4;
+  opacity: 0.8;
+}
+
+.openclaw-preview {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+  border-top: var(--border-width) solid var(--f1-border);
+  padding-top: 8px;
+}
+
+.openclaw-preview-msg {
+  font-size: var(--small-font-size);
+  display: flex;
+  gap: 6px;
+  align-items: flex-start;
+  line-height: 1.4;
+}
+
+.openclaw-preview-msg i {
+  flex-shrink: 0;
+  margin-top: 2px;
+  width: 14px;
+  text-align: center;
+}
+
+.openclaw-preview-user {
+  color: var(--text-color);
+}
+
+.openclaw-preview-assistant {
+  color: var(--muted);
+}
+
+.openclaw-preview-msg span {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}