Integrate XP.css for Windows XP aesthetic

- Add XP.css framework via CDN
- Restructure HTML with XP window components
- Use fieldsets for grouped controls
- Add classic window with title bar and status bar
- Replace dark mode with XP theme
- Update custom CSS to complement XP.css styling
- Use system colors (ButtonFace, Window, etc.)
- Add classic teal background
- Maintain all existing functionality

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

static/app.js

@@ -1,42 +1,4 @@
 (() => {
-  // Theme management
-  const themeToggle = document.getElementById("themeToggle");
-  const prefersDark = window.matchMedia("(prefers-color-scheme: dark)");
-
-  function getTheme() {
-    const saved = localStorage.getItem("theme");
-    if (saved) return saved;
-    return prefersDark.matches ? "dark" : "light";
-  }
-
-  function setTheme(theme) {
-    document.documentElement.setAttribute("data-theme", theme);
-    localStorage.setItem("theme", theme);
-    if (themeToggle) {
-      themeToggle.textContent = theme === "dark" ? "☀️ Light Mode" : "🌙 Dark Mode";
-    }
-  }
-
-  function toggleTheme() {
-    const current = document.documentElement.getAttribute("data-theme") || "light";
-    setTheme(current === "dark" ? "light" : "dark");
-  }
-
-  // Initialize theme
-  setTheme(getTheme());
-
-  // Listen for theme toggle
-  if (themeToggle) {
-    themeToggle.addEventListener("click", toggleTheme);
-  }
-
-  // Listen for system theme changes
-  prefersDark.addEventListener("change", (e) => {
-    if (!localStorage.getItem("theme")) {
-      setTheme(e.matches ? "dark" : "light");
-    }
-  });
-
   let qs = new URLSearchParams(window.location.search);
   const qInput = document.getElementById("q");
   const channelDropdown = document.getElementById("channelDropdown");