i18n(ai-chat): rename typing indicator to "Thinking…"

Replace the AI chat typing indicator text "AI is typing…" with
"Thinking…".

- typing-indicator.tsx: use t("Thinking…") instead of t("AI is typing…")
- en-US: drop the now-redundant "AI is typing…" key (the "Thinking…"
  key already existed and was unused)
- ru-RU: rename the key to "Thinking…" with value "Думаю…"
- update related comments in message-list.tsx and the test file

apps/client/src/features/ai-chat/components/message-list.tsx

@@ -43,7 +43,7 @@ interface MessageListProps {
 const BOTTOM_THRESHOLD = 40;
 
 /**
- * Whether to show the standalone "AI is typing…" indicator. It bridges every
+ * Whether to show the standalone "Thinking…" indicator. It bridges every
  * gap in a turn where the assistant is working but nothing visible is actively
  * being produced yet — so it shows while a turn is in flight AND the latest
  * assistant message's LAST part is not live output:

apps/client/src/features/ai-chat/components/show-typing-indicator.test.ts

@@ -5,7 +5,7 @@ import { showTypingIndicator } from "@/features/ai-chat/components/message-list.
 /**
  * Pure-helper tests for the typing-indicator bridging logic that the internal
  * chat and the public share widget now share. This is the behavior that decides
- * whether the animated "AI is typing…" placeholder shows in the gap
+ * whether the animated "Thinking…" placeholder shows in the gap
  * between sending and the first streamed token.
  */
 const msg = (

apps/client/src/features/ai-chat/components/typing-indicator.tsx

@@ -21,7 +21,7 @@ interface TypingIndicatorProps {
  * Mirrors the assistant row layout in MessageItem (the dimmed label), so it reads
  * as the assistant's bubble taking shape. The dimmed label uses the configured
  * identity name when provided (otherwise the generic "AI agent"), while the
- * typing line is always the generic "AI is typing…" (it never includes the
+ * typing line is always the generic "Thinking…" (it never includes the
  * role/identity name).
  */
 export default function TypingIndicator({ assistantName }: TypingIndicatorProps) {
@@ -40,7 +40,7 @@ export default function TypingIndicator({ assistantName }: TypingIndicatorProps)
           <span />
         </span>
         <Text size="sm" c="dimmed">
-          {t("AI is typing…")}
+          {t("Thinking…")}
         </Text>
       </Group>
     </Box>