docs(sessions): add captured screenshot for session start dropdown (PR G)

Captures the planned screenshot × 4 languages (4 PNGs):
- session_start_dropdown.png — Launcher's Launch dropdown + Multiple Sessions dialog with the Number of sessions input (FR-2324 / #6045)

Framing note: clicking "Launch Multiple Sessions" opens a dialog (not an inline dropdown input), which is what the doc prose actually describes; filename preserved per the existing doc reference.

Captured at 2× zoom via Portless dev, per-locale via window.switchLanguage().

Author: yomybaby

packages/backend.ai-webui-docs/src/en/images/session_start_dropdown.png

@@ -242,7 +242,6 @@ the resources shown on the form. The single-click 'Launch' action still starts
 one session by default.
 
 ![](../images/session_start_dropdown.png)
-<!-- TODO: Capture screenshot of the start button dropdown showing "Launch Multiple Sessions" -->
 
 :::note
 Concurrent session launch is capped by the system; if the requested total

packages/backend.ai-webui-docs/src/en/sessions_all/sessions_all.md

@@ -230,7 +230,6 @@ Backend.AI WebUIで最もよく利用されるページは、「セッション
 「ローンチ」ボタンを単にクリックした場合は、既定で 1 つのセッションのみが起動します。
 
 ![](../images/session_start_dropdown.png)
-<!-- TODO: Capture screenshot of the start button dropdown showing "複数セッションを起動" -->
 
 :::note
 同時セッション起動はシステムによって制限されます。要求された合計が残りのセッション

packages/backend.ai-webui-docs/src/ja/images/session_start_dropdown.png

@@ -159,7 +159,6 @@ Backend.AI WebUI 에서 가장 많이 방문하게 될 페이지는 세션 페
 값으로 한 개의 세션만 시작됩니다.
 
 ![](../images/session_start_dropdown.png)
-<!-- TODO: Capture screenshot of the start button dropdown showing "복수 세션 동시 시작" -->
 
 :::note
 동시 세션 생성은 시스템 정책에 의해 제한됩니다. 요청한 총 세션 수가 남은 동시 세션

packages/backend.ai-webui-docs/src/ja/sessions_all/sessions_all.md

@@ -223,7 +223,6 @@
 จะเริ่มเซสชันเพียงหนึ่งเซสชันตามค่าเริ่มต้น
 
 ![](../images/session_start_dropdown.png)
-<!-- TODO: Capture screenshot of the start button dropdown showing "เริ่มหลายเซสชัน" -->
 
 :::note
 การเปิดเซสชันพร้อมกันถูกจำกัดโดยระบบ หากจำนวนรวมที่ขอเกินโควตาเซสชันที่เหลืออยู่