Sync from upstream llama.cpp repository

tools/server/webui/docs/flows/data-flow-simplified-model-mode.md

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+```mermaid
+%% MODEL Mode Data Flow (single model)
+%% Detailed flows: ./flows/server-flow.mmd, ./flows/models-flow.mmd, ./flows/chat-flow.mmd
+
+sequenceDiagram
+    participant User as 👤 User
+    participant UI as 🧩 UI
+    participant Stores as 🗄️ Stores
+    participant DB as 💾 IndexedDB
+    participant API as 🌐 llama-server
+
+    Note over User,API: 🚀 Initialization (see: server-flow.mmd, models-flow.mmd)
+
+    UI->>Stores: initialize()
+    Stores->>DB: load conversations
+    Stores->>API: GET /props
+    API-->>Stores: server config + modalities
+    Stores->>API: GET /v1/models
+    API-->>Stores: single model (auto-selected)
+
+    Note over User,API: 💬 Chat Flow (see: chat-flow.mmd)
+
+    User->>UI: send message
+    UI->>Stores: sendMessage()
+    Stores->>DB: save user message
+    Stores->>API: POST /v1/chat/completions (stream)
+    loop streaming
+        API-->>Stores: SSE chunks
+        Stores-->>UI: reactive update
+    end
+    API-->>Stores: done + timings
+    Stores->>DB: save assistant message
+
+    Note over User,API: 🔁 Regenerate
+
+    User->>UI: regenerate
+    Stores->>DB: create message branch
+    Note right of Stores: same streaming flow
+
+    Note over User,API: ⏹️ Stop
+
+    User->>UI: stop
+    Stores->>Stores: abort stream
+    Stores->>DB: save partial response
+```