Add configurable taskbar placement

[rimtty]

Summary

• add a taskbar placement selector to QuickPods Settings
• keep Left of notification area as the default placement
• provide Left side of taskbar as the alternative legacy placement
• persist the setting, propagate it through the taskbar-host IPC protocol, and re-place the surface immediately when it changes
• retain Main's application update settings and update-check UI while integrating the new Settings row
• update English and Japanese documentation and diagnostics

Why

The notification-area-adjacent placement was validated successfully as an experimental branch. This PR promotes it into the Main integration path while preserving the existing taskbar-left behavior as a user-selectable option.

Main integration notes

The branch was merged on top of origin/main at 853aab5. Conflicts in QuickPodsSettings and its JSON persistence tests were resolved by retaining both the application-update fields from Main and the new taskbar-placement field. The Settings UI was reviewed to ensure the update controls and taskbar-placement selector coexist.

Validation

• dotnet restore QuickPods.sln --locked-mode
• dotnet build QuickPods.sln -c Release --no-restore — 0 warnings, 0 errors
• dotnet test tests/QuickPods.Foundation.Tests/QuickPods.Foundation.Tests.csproj -c Release --no-restore — 149 passed
• solution-wide test run — 448 passed, 1 pre-existing Spike failure (Rapid_click_to_jump_updates_present_expected_frames_without_GDI_growth, USER handle baseline 13 → 31), reproduced before this integration