dovenest

Redesign UI: Material Symbols icons, full-width topbar + sidebar, themes

Icons:
- Custom SVG favicon (blue square, folder with chevron cut-out), reused as
  the inline Logo component in topbar/auth cards/share page
- Central icon registry (web/src/icons.rs): all glyphs are Material
  Symbols (filled, 24x24) from the iconify.design API, each variant's
  doc comment names the source glyph so any swap is a one-file change
- Replaces every hand-drawn SVG and emoji (including the old admin crown)

Layout:
- Sharp squares everywhere (global border-radius: 0)
- Full-width sticky topbar (logo left, username + theme toggle + logout
  right); sidebar (FILES root tabs + MANAGE: Shares/Users/Settings) is
  docked beneath it on wide screens and an off-canvas drawer under the
  topbar on narrow ones, so the topbar stays interactable
- Accessible roots are sidebar tabs; the virtual root-picker view is gone
- Shares is a full view (no overlay); user rows no longer show accessible
  folders (edit modal only); admin badge removed
- Context menu items get icons; Refresh moves from the menu to a
  toolbar icon button

Themes:
- Tri-state toggle next to logout: auto (default, follows the OS) /
  light / dark, persisted in localStorage, applied as a class on <html>
  (app.css resolves the palettes); a pre-paint script in index.html
  avoids flashing the wrong palette; color-scheme set per theme

Fixes:
- .center-screen was a row flex, rendering the share error heading and
  message side by side; now a column
- just dev-web: trunk strips the /api rewrite prefix, so the proxy
  backend URL now carries it (login was broken in dev)
- embedded frontend e2e: whitelist the svg favicon content-type

Co-Authored-By: Qwen3.8 27b