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/model-selector.tsx

@@ -220,14 +220,14 @@ export function ModelSelector({
                             <span className="min-w-0 truncate text-xs">
                                 {selectedModel
                                     ? shortModelName(selectedModel.modelId)
-                                    : dict.modelConfig.default}
+                                    : dict.modelConfig.serverDefault}
                             </span>
                         ) : (
                             // Keep an sr-only label for screen readers when hidden
                             <span className="sr-only">
                                 {selectedModel
                                     ? selectedModel.modelId
-                                    : dict.modelConfig.default}
+                                    : dict.modelConfig.serverDefault}
                             </span>
                         )}
                         <ChevronDown className="h-3 w-3 flex-shrink-0 text-muted-foreground" />