UI polish: spacing, shadows, and Folio theme token adjustments

[kaihuang1425]

Motivation

• Refine the Waypoint-style new-tab UI with improved spacing, sizing, and visual polish across sidebar, rail, topbar, and home surface.
• Replace hardcoded, theme-colored shadow casts with neutral chrome shadow tokens so chrome elevation reads consistently across themes.
• Tweak the default Folio palette and shadow strengths to produce a softer, more balanced light signature theme.
• Keep theme defaults in sync between CSS and JS so the initial UI matches the updated tokens.

Description

• Introduced theme-neutral chrome elevation variables (--chrome-edge-shadow, --chrome-side-shadow-r, --chrome-side-shadow-l) in newtab.css and added per-theme overrides for folio, light, and tabento.
• Replaced hardcoded box-shadows on #sidebar, #tabrail and #topbar with the new chrome shadow variables and adjusted several sizing tokens (--sidebar, --topbar, --tabrail) for gentler scaling.
• Multiple spacing, padding and radius refinements across the UI (sidebar, brand lockup, topbar search, rail header/footer, home hero and cards), plus improvements to grid sizing (minmax(min(330px, 100%), 1fr)), borders and color-mix usage for subtler strokes.
• Adjusted the Folio light theme palette and shadow tokens in themes.css and updated the default folio color value in newtab.js to match the new palette; removed brand gradient text clipping from the brand name for consistent text rendering.

Testing

• No automated tests were executed for this change.