feat(MilkdownEditor): add file upload support for documents and text files

Added new upload functionality to the editor supporting doc/docx/ppt/pptx/pdf/zip/txt/json files. Includes:
- New upload button with file input
- File type detection utilities (isTextFile, isConvertibleFile)
- Initial markdown sync with trailing whitespace normalization
- Warning messages for unsupported file types

src/stores/settings.js

@@ -45,6 +45,11 @@ export const useSettingsStore = defineStore('settings', () => {
         return translations[uiLanguage.value] || translations['en']
     })
 
+    const initialMarkdown = computed(() => {
+        const lang = uiLanguage.value
+        return translations[lang]?.initialMarkdown || translations['en']?.initialMarkdown || ''
+    })
+
     // --- Actions/Logic ---
 
     // Load from localStorage
@@ -138,6 +143,7 @@ export const useSettingsStore = defineStore('settings', () => {
         backgroundOpacity,
         uiLanguage,
         t,
+        initialMarkdown,
         resetSettings
     }
 })