refactor(chat): deduplicate streaming code, add multi-conv, and XSS protection

- Add ChatEngine for deduplicated chat logic (handlers_chat/shell_chat)
- Add SendWithToolsStream for real-time streaming responses
- Add /help, /plan, /export, /model commands in Studio
- Fix XSS: sanitize HTML after markdown rendering
- Add ConversationStoreMulti for multi-conversation support
- Add Anthropic headers (x-api-key, anthropic-version)
- Add fallback logging when provider switch occurs
- Add API handler tests (handlers_test.go)
- Polish Studio: max-height 200px, word-break on tool args
- Update CLI version to show full info (version, go, platform)

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Assisted-by: MiniMax-M2.5 via Crush <crush@charm.land>

web/src/styles/global.css

@@ -684,6 +684,8 @@ input::placeholder { color: var(--text-disabled); }
   background: var(--bg-surface); border: 1px solid var(--border); border-left: 2px solid var(--accent-dim);
   border-radius: var(--radius); margin: 6px 0 8px; overflow: hidden;
   transition: all 0.3s ease;
+  max-height: 200px;
+  overflow-y: auto;
 }
 .feed-thinking-block.active {
   border-left-color: var(--warning);
@@ -826,7 +828,8 @@ input::placeholder { color: var(--text-disabled); }
   font-size: 12px;
   font-family: var(--font-mono);
   color: var(--text-tertiary);
-  white-space: nowrap;
+  white-space: pre-wrap;
+  word-break: break-all;
   overflow: hidden;
   text-overflow: ellipsis;
   border-bottom: 1px solid var(--border);