Fix tray popup placement for vertical taskbars

[Finesssee]

Fixes #121.

Summary

• Infer tray fallback anchors from the taskbar side instead of assuming bottom-right.
• Reuse the same inferred anchor when the frontend re-anchors after resizing.
• Add left/right vertical taskbar regressions.

Local checks

• cargo test --manifest-path apps\desktop-tauri\src-tauri\Cargo.toml
• git diff --check
• Cua Driver CLI desktop/screen inspection

[Finesssee]

Local built UI verification is done with Cua Driver on Windows 10 using a left-docked taskbar.

Result after the tray-click fix: CodexBar opens at x=132,y=674,width=328,height=327, anchored beside the left taskbar (work_area={X=124,Y=0,Width=1796,Height=1080}), instead of the old bottom-right popout position (x=1492,y=352,width=420,height=680). Restored the taskbar back to bottom after the test.

[Finesssee]

Thermo-nuclear code quality review complete.

Found one structural cleanup issue before merge: commands/system.rs was reaching into shell::geometry and manually constructing MonitorPlacement, which leaked the shell geometry internals into the command layer. I fixed that in 06643db3 by keeping geometry private again and exposing a shell-level reanchor positioning helper instead.

After that cleanup, I don't see remaining blockers: the taskbar-side inference lives in the canonical shell positioning layer, the resize re-anchor path reuses the same positioning policy, the tray-click route now targets TrayPanel instead of PopOut, and the 1k+ tray_bridge.rs file was already over the threshold before this PR rather than being pushed over by it.