feat: add theme management with light and dark modes

- Implemented a new composable `useTheme` for managing theme state.
- Added functions to read and write theme preference to local storage.
- Applied theme styles to the DOM based on user preference.
- Introduced a toggle function to switch between light and dark themes.

refactor: enhance copilot plugin functionality

- Improved request handling with sequence and document versioning.
- Refactored ghost text handling to improve clarity and efficiency.
- Updated markdown insertion logic to handle parsed content more robustly.
- Enhanced error handling and logging for better debugging.

style: update global styles for light and dark themes

- Defined CSS variables for light and dark themes to streamline styling.
- Improved overall styling consistency and responsiveness.
- Added transitions for smoother theme changes and interactions.

src/composables/useTheme.js

@@ -0,0 +1,70 @@
+import { computed, ref } from 'vue'
+
+const STORAGE_KEY = 'llm-in-text.theme'
+const DEFAULT_THEME = 'light'
+const THEME_LIGHT = 'light'
+const THEME_DARK = 'dark'
+
+const theme = ref(DEFAULT_THEME)
+
+const canUseDom = typeof document !== 'undefined'
+const canUseStorage = typeof window !== 'undefined' && typeof window.localStorage !== 'undefined'
+
+const normalizeTheme = (value) => (value === THEME_DARK ? THEME_DARK : THEME_LIGHT)
+
+const readStoredTheme = () => {
+  if (!canUseStorage) return DEFAULT_THEME
+
+  try {
+    const stored = window.localStorage.getItem(STORAGE_KEY)
+    return normalizeTheme(stored)
+  } catch {
+    return DEFAULT_THEME
+  }
+}
+
+const writeStoredTheme = (value) => {
+  if (!canUseStorage) return
+
+  try {
+    window.localStorage.setItem(STORAGE_KEY, value)
+  } catch {
+    // Ignore persistence errors (privacy mode, quota, etc).
+  }
+}
+
+const applyThemeToDom = (value) => {
+  if (!canUseDom) return
+
+  const next = normalizeTheme(value)
+  document.documentElement.setAttribute('data-theme', next)
+  document.documentElement.style.colorScheme = next
+}
+
+const setTheme = (nextTheme) => {
+  const next = normalizeTheme(nextTheme)
+  theme.value = next
+  applyThemeToDom(next)
+  writeStoredTheme(next)
+}
+
+const toggleTheme = () => {
+  setTheme(theme.value === THEME_DARK ? THEME_LIGHT : THEME_DARK)
+}
+
+const isDark = computed(() => theme.value === THEME_DARK)
+
+const initTheme = () => {
+  setTheme(readStoredTheme())
+}
+
+initTheme()
+
+export function useTheme() {
+  return {
+    theme,
+    isDark,
+    setTheme,
+    toggleTheme
+  }
+}