Document Looking Glass fullscreen activation

[amariichi]

Summary

• Add the Looking Glass presentation-window double-click/full-screen step to the English and Japanese startup instructions.
• Add the same check to both troubleshooting sections when the presentation does not show the correct 3D view.

The instruction identifies the presentation window itself as the double-click target, after it has been placed on the Looking Glass display.

Validation

• Backend: 47 tests passed.
• Frontend: 105 tests passed.
• git diff --check: passed.
• English and Japanese instructions use the same step order and behavior.

The existing v1.2.0 tag will not be moved. After this PR merges, the published v1.2.0 Release notes will receive the matching Platform note.