Polish: tours, toolbar, introduction, sheets, explanations, in-app links !41

1. Tours: no Next on the last stop.
2. Tours: Next/Back pan the view over about a second, the short way round (stepped by hand, since the sky is a Canvas). A drag cancels the pan.
3. Tours: the target is highlighted: a constellation's lines drawn bright and thick, a star or body ringed (SkyHighlight in SkyRenderer; red in night mode). The tour card is now opaque.
4. Sky toolbar: the six icons are one labelled Explore menu (Tonight: tours, events, point the phone; Explore: another year, observing log; night mode, settings). A chip shows while the view follows the phone, with Stop.
5. Introduction: five pages (welcome, the Sky tab, the other tabs, Stay in the loop, location) with Skip. Stay in the loop offers the daily summary, aurora alerts and space news, asking for notification permission only when one is turned on. Settings › Show the introduction again.
6. Year sheet: large year readout, slider across 3000 BCE–3000 CE, ±1/±100 steps, live pole star, moments list. Events: app-style header and intro, date column, pinned month headers, visibility dot, chevron on solar eclipses. Both drop the blue tint for the app's red.
7. System and Sun: an intro under each title, and a one-line note under each section (spacecraft, DSN, Mars, aurora, flares, Kp) saying what it shows and why it matters.
8. Every http(s) link opens in Safari's in-app browser over the current screen (InAppBrowser via the openURL environment), including from sheets.

Checked on the simulator: all eight. Package tests pass; visionOS builds.