fix(Select): guard empty results and retain modal popup scope - #752

Summary

Addresses the shared Select defects tracked in #753 and reported in mieweb/eSheet#256:

• Safely handle empty search results for ArrowDown, ArrowUp, Home, End, and Enter without emitting an invalid selection.
• Keep popup DOM descendants within dialog/alertdialog focus and accessibility scope; use the browser top layer to preserve positioning in transformed dialogs.
• Re-query modal focus boundaries on Tab so dynamically opened search controls stay contained.
• Expose active options through aria-activedescendant on the trigger and searchable input, with matching consumer-facing documentation.
• Set explicit popup foreground colors and correct the dark-theme browser regression's token overrides to take precedence over Storybook brand styles.

Validation

• 37 Select unit tests passed, including no-match navigation, active-option associations, real-Modal Tab/Shift+Tab containment, and alertdialog scope.
• pnpm typecheck and full pnpm lint passed.
• Prettier validation and formatting passed for the three follow-up files.
• Integrated-browser assertions passed for dark popup/option colors, focused search within the dialog, and transformed-modal alignment (approximately 0.005 px error).
• The CLI Playwright runner could not launch Chrome in this environment; the relevant browser assertions were executed through the integrated browser instead.

Full downstream eSheet compose/upload integration and assistive-technology testing were not repeated during this follow-up. No npm release has been published by this update.