fix: use closest attachment index for context menu download

[fresh3nough]

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Description

Fixes #1828

Summary:

• Use the nearest data-attachmentindex ancestor when opening the message context menu so nested targets (e.g., the play icon) map to the correct attachment.
• Add a unit test that exercises nested attachment targets.

Repro steps (pre-fix):

1. Open Session Desktop 1.17.8 on Ubuntu.
2. Receive a message with multiple video attachments.
3. Right-click the play icon on the second video and choose Save.
4. The downloaded file is the first video.

Test plan:

• Pre-fix: pnpm exec mocha --require jsdom-global/register app/ts/test/components/MessageContextMenu_test.js (fails: expected 0 to equal 2)
• Post-fix: pnpm exec mocha --require jsdom-global/register app/ts/test/components/MessageContextMenu_test.js
• pnpm ready

Manual testing:

• Not run (CLI-only environment).

Co-Authored-By: Warp agent@warp.dev