feat(admin-ui): softer pre-material styling and dark theme

- Rebuild admin console styling on semantic CSS variables (light + dark palettes)
- Soften palette, shadows and corner radii for a calmer pre-material look
- Add theme provider with light/dark toggle in the topbar and login header
- Respect prefers-color-scheme and persist choice, with anti-FOUC inline script
- Polish Russian translations (yo letters, consistent 'Звёзды' currency, wording)
- Rebuild dist bundle to match the updated source

cmd/telesrv-admin/web/src/theme.tsx

@@ -0,0 +1,106 @@
+import { createContext, useCallback, useContext, useEffect, useMemo, useState, type ReactNode } from "react";
+import { Moon, Sun } from "lucide-react";
+import { useI18n } from "./i18n";
+
+export type Theme = "light" | "dark";
+
+const storageKey = "telesrv.admin.theme";
+
+type ThemeContextValue = {
+  theme: Theme;
+  setTheme: (theme: Theme) => void;
+  toggleTheme: () => void;
+};
+
+const ThemeContext = createContext<ThemeContextValue | null>(null);
+
+export function applyTheme(theme: Theme) {
+  document.documentElement.setAttribute("data-theme", theme);
+  document.documentElement.style.colorScheme = theme;
+}
+
+export function ThemeProvider({ children }: { children: ReactNode }) {
+  const [theme, setThemeState] = useState<Theme>(() => initialTheme());
+
+  useEffect(() => {
+    applyTheme(theme);
+    try {
+      localStorage.setItem(storageKey, theme);
+    } catch {
+      // Theme persistence is best-effort.
+    }
+  }, [theme]);
+
+  // Follow the OS preference until the user makes an explicit choice.
+  useEffect(() => {
+    if (!window.matchMedia) {
+      return;
+    }
+    const media = window.matchMedia("(prefers-color-scheme: dark)");
+    const onChange = (event: MediaQueryListEvent) => {
+      let stored: string | null = null;
+      try {
+        stored = localStorage.getItem(storageKey);
+      } catch {
+        stored = null;
+      }
+      if (stored !== "light" && stored !== "dark") {
+        setThemeState(event.matches ? "dark" : "light");
+      }
+    };
+    media.addEventListener("change", onChange);
+    return () => media.removeEventListener("change", onChange);
+  }, []);
+
+  const setTheme = useCallback((next: Theme) => setThemeState(next), []);
+  const toggleTheme = useCallback(() => setThemeState((current) => (current === "dark" ? "light" : "dark")), []);
+
+  const value = useMemo<ThemeContextValue>(() => ({ theme, setTheme, toggleTheme }), [theme, setTheme, toggleTheme]);
+
+  return <ThemeContext.Provider value={value}>{children}</ThemeContext.Provider>;
+}
+
+export function useTheme(): ThemeContextValue {
+  const value = useContext(ThemeContext);
+  if (!value) {
+    throw new Error("useTheme must be used inside ThemeProvider");
+  }
+  return value;
+}
+
+export function ThemeSwitch() {
+  const { theme, toggleTheme } = useTheme();
+  const { t } = useI18n();
+  const nextIsDark = theme === "light";
+  const label = t(nextIsDark ? "theme.switchToDark" : "theme.switchToLight");
+  return (
+    <button
+      className="theme-toggle"
+      type="button"
+      onClick={toggleTheme}
+      aria-label={label}
+      title={label}
+    >
+      {theme === "dark" ? <Sun size={16} /> : <Moon size={16} />}
+    </button>
+  );
+}
+
+function initialTheme(): Theme {
+  try {
+    const stored = localStorage.getItem(storageKey);
+    if (stored === "light" || stored === "dark") {
+      return stored;
+    }
+  } catch {
+    // Storage is optional.
+  }
+  try {
+    if (window.matchMedia && window.matchMedia("(prefers-color-scheme: dark)").matches) {
+      return "dark";
+    }
+  } catch {
+    // matchMedia can be unavailable in unusual embedded contexts.
+  }
+  return "light";
+}