Add native Windows caption controls

[xptea]

Summary

• add native Windows caption controls to Comet's unified titlebar
• expose the controls across loading, authentication, organization, error, settings, and chat views
• reserve titlebar space so tabs and settings content do not overlap the controls

Why

Windows used a frameless titlebar but did not render minimize, maximize/restore, or close controls. This made the window difficult to manage and left pre-login views without window chrome.

Validation

• cargo build -p comet
• cargo test -p comet-ui --lib shell::tests:: (18 passed)
• cargo test -p comet-ui --lib (357 passed; existing Windows IPC port-release test failed)
• manual verification on Windows, including loading and sign-in views

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[wingleeio]

Are you able to provide video/images of how this looks on windows?

[xptea]

Are you able to provide video/images of how this looks on windows?

Sure! This change adds native window decorations to every page on Windows. Previously, the window was frameless, so there were no standard controls to minimize, maximize, or close it.

[wingleeio]

Reviewed the diff: pure UI change, no dependency/CI/build-script modifications, no unsafe or Win32 calls, and a byte-level scan found no invisible Unicode. Safe to merge.