fix(gui): render Linux popup after activation

[StudentWeis]

Summary

Fix Linux/X11 popup activation so Ropy starts hidden and explicitly invalidates the GPUI board before mapping the popup. This prevents a mapped-but-unpainted window during the first hotkey or tray activation.

Linked Issue

Closes #163

Changes

• Hide the Linux X11 window after native window discovery instead of requesting startup activation.
• Focus and notify the board entity before restoring the popup.
• Add a GPUI regression test that observes the activation notification required for the first visible frame.
• Enable GPUI test support for window-level component tests.

Testing

• scripts/precheck.sh passes locally
• New / updated tests cover the change
• cargo test passes with 472 tests

Self-Check

• PR title follows Conventional Commits
• No hardcoded user-facing strings — no user-facing strings added
• Errors defined with thiserror — no new error types added
• No unrelated changes mixed in