Describe the v2026.10.17-beta.1 preview in the changelog and README

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Prepares the next preview release.

• CHANGELOG: add v2026.10.17-beta.1 with the user-facing changes since v2026.10.10-beta.1, including those first shipped in v2026.10.13-beta.1: groups across apps, remembered groups, virtual-desktop grouping, workspace restores that launch missing apps, text selection across permission levels, taskbar thumbnail and icon fixes, vertical tabs, the Close tab shortcut and the smaller exe.
• README: update the preview banner, list Ctrl + Alt + W for closing a tab, and describe Groups across apps and launching missing apps on workspace restore.