feat(ui): tidy the start screen and the chat panel details

Start screen
- The five examples are one row of picture tiles (title under the picture); the old cards cut their descriptions off with an ellipsis
- "Open .drawio file" leaves the example grid and sits with "Start with a blank canvas" below it, both as outlined buttons
- The "Paper to diagram" example gets a thumbnail of its result (public/paper_demo.svg) like the other four
- The caption says what happens: examples use saved answers, so they appear at once and do not call the AI
- Shorter lede

Chat panel
- The version card's "</>" icon button becomes a "Show XML" text link in the summary line; the summary runs under the title row, so Undo is the card's only button
- Settings uses the gear icon; new diagram uses a file-plus icon instead of the pen that reads as "edit"
- The message box has the same side padding as the message column

Copy
- The model picker says "Server model" instead of "Default" when the server's model is in use
- The theme description no longer claims the diagram always stays on a white sheet (the canvas follows the theme)
- en, zh, ja and zh-Hant dictionaries updated together

components/workspace/workspace.tsx

@@ -1,6 +1,6 @@
 "use client"
 
-import { MessageSquare, PanelRightOpen, Settings2, Shapes } from "lucide-react"
+import { MessageSquare, PanelRightOpen, Settings, Shapes } from "lucide-react"
 import { usePathname, useRouter } from "next/navigation"
 import { useEffect, useRef, useState } from "react"
 import { CanvasStage } from "@/components/canvas/canvas-stage"
@@ -100,7 +100,7 @@ function StartBar({
                 onClick={() => openSettings()}
                 data-testid="settings-button"
             >
-                <Settings2 />
+                <Settings />
             </IconButton>
         </div>
     )