feat(shellv2): handle late beacon checkin in UI

[KCarretto]

This PR updates the ShellV2 interface to visually indicate when a beacon is more than 5 minutes late to check in. It greys out the terminal text and disables user input to prevent confusion or wasted effort on a disconnected session.

Changes:

• Modified useCallbackTimer hook to calculate isLateCheckin status based on a 5-minute threshold.
• Updated useShellTerminal hook to accept isLateCheckin status.
• Implemented logic in useShellTerminal to dynamically change the xterm theme (grey foreground) and block input events when the beacon is late.
• Added a unit test suite for useCallbackTimer to verify time calculation logic.

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PR created automatically by Jules for task 18014522677988305833 started by @KCarretto

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[github-actions]

Summary

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
2686    ±0	2686    ±0	0    ±0	0    ±0	0    ±0	0    ±0	1ms    ±0

Previous Results

Build 🏗️	Result 🧪	Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
#1298	✅	2686	2686	0	0	0	0	30.1s

Insights

Average Tests per Run	Total Flaky Tests	Total Failed	Slowest Test (p95)
2686	0	0	6.2s

Slowest Tests

Test 📝	Results 📊	Duration (avg) ⏱️	Duration (p95) ⏱️
TestDockerExecutor_Build_ContextCancellation	1	6.2s	6.2s
eldritch-libsys: std::dll_inject_impl::tests::test_dll_inject_simple	1	5.1s	5.1s
TestInteractiveShell	1	5.0s	5.0s
TestOtherStreamOutput	1	5.0s	5.0s
imix::bin/imix: install::tests::test_install_execution	3	1.6s	4.6s
imix::bin/imix: install::tests::test_install_execution	3	1.6s	4.6s
imix::bin/imix: install::tests::test_install_execution	3	1.6s	4.6s
TestDockerExecutor_Build_SimpleEcho	1	3.4s	3.4s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s

_{🎉 No failed tests in this run. | 🍂 No flaky tests in this run.}

Github Test Reporter by CTRF 💚

🔄 This comment has been updated