Share the popover CSS between the tour and hints entries

The popover styling moves to src/popover.css, which both driver.css and
hints.css @import. Vite inlines the import at build time, so each entry's
stylesheet is self-contained: driver.css keeps the same rule set it had
before (verified against master — identical set, just regrouped), and
hints.css now carries the popover rules too.

This closes a gap the playground surfaced: hints.css previously held only
the beacon styles, so a hints-only consumer importing it got an unstyled
popover. Now importing hints.css is all they need. Consumers of both
entries ship the popover rules twice (~0.6KB gzip), which is harmless
since the two copies are identical.

custom-buttons.test.ts injected driver.css raw to assert the footer button
computed styles; jsdom doesn't follow @import, so it now injects
popover.css, where those rules live.