The tab's icon follows light and dark mode (#64)

The logo on the page switched with the mode (#63), but the tab's icon
was always favicon.png, the light logo. web/favicon-dark.png is
logo-dark.svg at 128px, served beside it, and the theme script points
the icon link at whichever matches data-mode, so it follows the theme
the account chose, not only the system. The sign-in page, with no
account, picks by the system's with two media-bound links.
/favicon.ico, which a browser asks for on its own, stays the light one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/theme.ts

@@ -35,6 +35,8 @@ function setTheme(name = theme.name, mode = theme.mode, save = false){
   // A theme with one palette has it whatever the mode; both of those are light.
   root.dataset.mode = !both ? 'light'
     : theme.mode === 'auto' ? (systemDark && !systemDark.matches ? 'light' : 'dark') : theme.mode;
+  // The tab's icon in the same variant as the logo on the page.
+  const fav = $('#favicon'); if(fav) fav.href = fav.dataset[root.dataset.mode];
   const sel = $('#stheme'); if(sel) sel.value = theme.name;
   const ms = $('#smode'); if(ms) ms.value = theme.mode;
   const mf = $('#smodefield'); if(mf) mf.hidden = !both;