[AmrMsCLL]

Adds shared motion so apps feel smoother, with no new dependency. Everything is off under prefers-reduced-motion: reduce.

• Page changes: new PageTransition export, a React 19.3 ViewTransition keyed by the page. The old page fades out, the root snapshot doesn't animate, so the header, sidebar and tab bar stay still. .page-enter is now a 4px rise as well as a fade.
• Moving highlights: the Tabs underline morphs to the new tab across pages through a named view transition. The sidebar's active background is a single indicator that slides to the new row. The phone tab bar gets a pill behind the active icon that slides between tabs.
• Header: its border and shadow appear once the page scrolls (scroll-driven where supported, otherwise the border stays as before).
• Overlays: Popover and Menu default to Fade plus a small grow from the anchor, instead of MUI's Grow. Autocomplete lists get the same entrance. Dialogs (including CommandPalette) and snackbars rise in. MUI enter and leave durations are 220ms and 160ms.
• Controls: buttons, icon buttons and clickable chips scale to 0.97 while pressed. Checkbox pops when the user checks it; it marks itself data-pressed on its first change, so pages don't pop every checked box on load.
• Content: <details> opens and closes with a height animation. .panel-rows children fade in (opacity only, so skeleton swaps stay seamless). A new opt-in .stagger-in class cascades its children.

Breaking: the React peer is now ^19.3.0, the first version that exports ViewTransition.

Checked in Strata against a packed build: lint, build, and in the browser the view transition, tab underline morph, sidebar and tab bar slides, header, menu, dialog, palette, snackbar and habit animations.