fix(hig): draw the switcher's highlighted row with its accent fill

[datlechin]

Why

The highlighted row in Switch Database showed white text on a light grey fill, close to unreadable. It happened on every open, and in Switch Connection after a search that matched nothing.

In a popover AppKit draws a row's selection with a material view and configures it only when the row's stored isEmphasized changes. The row answered emphasis from its getter, so the stored value never changed and the fill kept the look it got before the row had a window, while the cells turned white.

Change

• The highlighted row in the database, schema and connection switchers draws the accent fill with white content while its window is key, and the grey fill with standard colors when it is not.
• The row stores its emphasis through the setter and follows the window's key notifications, so fill and content change together.

Verified

Five tests in FieldDrivenListEmphasisTests fail on main. The two selection-material tests are skipped on the macOS 26 runner, which builds no selection material for a row outside a window; the stored-emphasis test covers that order there. Checked on a Debug build on macOS 27 in both switchers, the floating panel and on app deactivation. No UI test: the fill is not an accessibility attribute.

Screenshots

	Light	Dark
Before
After

Not in this PR

• Switch Database pressed over Open Quickly, after a chooser closed by itself, closes Open Quickly and opens nothing: the presenter keeps a stale subject.
• The fill was measured on macOS 27 only; whether UI: Incorrect selected state style #3249 shows on macOS 26 is unknown. A popover that opens with its rows already inside relies on the table's own emphasis write.

Fixes #3249