Step UIGAP: Desktop Compose Shell Rebuild

[deku2026]

Summary

• Rebuild the remaining UIGAP visual evidence closure for the Compose desktop shell.
• Keep ArcChat as full-window native desktop UI while recording the accepted Windows title-bar difference from NextChat browser captures.
• Add NextChat/ArcChat screenshot pairs and close UIGAP gap inventory, state matrix, and comparison notes.

Validation

• git -C C:\MyFile\DevAll\QmlSharp\NextChat rev-parse HEAD -> 89b8f26ff8f03a0c5b98fc3026d980721495227e
• ./gradlew.bat :apps:desktop:compileKotlin :desktop-shared:compose-ui:compileKotlin :desktop-shared:theme:compileKotlin passed
• ./gradlew.bat :apps:desktop:test :desktop-shared:compose-ui:test :desktop-shared:theme:test --stacktrace passed
• ./gradlew.bat :apps:desktop:test --stacktrace passed
• git diff --check passed

Traceability

• Updates docs/coverage/visual-baseline/uigap/GAP-INVENTORY.md
• Updates docs/coverage/visual-baseline/uigap/STATE-MATRIX.md
• Adds docs/coverage/visual-baseline/uigap/COMPARISON.md
• Adds docs/coverage/visual-baseline/uigap/{reference,arcchat}/ captures

Design Conflicts

• None. The outer browser/screenshot stage is explicitly not implemented in ArcChat; the native UI fills the desktop window.