feat(webui): brand logo and favicon

[aniongithub]

Gives the webui a real visual identity — a brand-blue chip-glyph that appears as the favicon, as a muted decoration in every page header, and front-and-center in the empty "select a page" state.

What's in the box

• webui/src/assets/favicon.svg — the chip glyph, baked with the brand accent color (#0078d4), tightened viewBox so it fills the favicon cell. Wired into index.html via HtmlWebpackPlugin's favicon option.
• webui/src/Logo.tsx — the same artwork as a Preact component with fill="currentColor", so wherever the logo is rendered it inherits the surrounding text color and works in both light and dark themes without per-theme assets.
• App.tsx + styles.css:
  • .page-header-logo — absolutely positioned, 72-px, var(--fg-dim) at opacity: 0.6, pointer-events: none. Sits in the top-right corner of every page header as a quiet brand mark.
  • .empty (no-page selected) — flex-column layout placing a 96-px logo above the "select a page" text, both inheriting var(--fg-dim) at opacity: 0.6 to match the page-header treatment.

Notes

• All three placements (favicon, page-header decoration, empty-state) draw the same paths from the same source; the favicon hardcodes the brand blue, the JSX uses currentColor so light/dark just works.
• pointer-events: none on the page-header logo so it never intercepts clicks on the header.
• The earlier iteration's full-detail original artwork (4 paths, fine dots, white stroke) didn't shrink to 16×16 cleanly — this simplified two-path chip block reads at any size and keeps the lineage with the larger placements.