fix(desktop): session persistence — refresh/login/context/empty-content 4-bug fix

1. App.tsx: add restoreSession() call on startup to prevent redirect
   to login page after refresh (isRestoring guard + BootstrapScreen)
2. CloneManager: call syncAgents() after loadClones() to restore
   currentAgent and conversation history on app load
3. zclaw-memory: add get_or_create_session() so frontend session UUID
   is persisted directly — kernel no longer creates mismatched IDs
4. openai.rs: assistant message content must be non-empty for
   Kimi/Qwen APIs — replace empty content with meaningful placeholders

Also includes admin-v2 ModelServices unified page (merge providers +
models + API keys into expandable row layout)

crates/zclaw-kernel/src/kernel.rs

@@ -645,15 +645,11 @@ impl Kernel {
         // Reuse existing session or create new one
         let session_id = match session_id_override {
             Some(id) => {
-                // Verify the session exists; if not, create a new one
-                let existing = self.memory.get_messages(&id).await;
-                match existing {
-                    Ok(msgs) if !msgs.is_empty() => id,
-                    _ => {
-                        tracing::debug!("Session {} not found or empty, creating new session", id);
-                        self.memory.create_session(agent_id).await?
-                    }
-                }
+                // Use get_or_create to ensure the frontend's session ID is persisted.
+                // This is the critical bridge: without it, the kernel generates a
+                // different UUID each turn, so conversation history is never found.
+                tracing::debug!("Reusing frontend session ID: {}", id);
+                self.memory.get_or_create_session(&id, agent_id).await?
             }
             None => self.memory.create_session(agent_id).await?,
         };