fix: DM message ordering, consolidate input toolbar, add rich text/WYSIWYG, file upload with drag-drop

- Fix DM backend ListMessages to use DESC + reverse (match channel handler)
- Remove spurious .reverse() from frontend message/conversation stores
- Create shared MessageInput component with Slack-style single toolbar row
- Add file upload via + button with progress bar and drag-and-drop
- Add markdown/rich text toggle with full WYSIWYG block formatting
  (lists, blockquotes, links, headings, code blocks)
- Add frontend+backend security for file uploads (extension + content-type guards)

web/src/stores/message.ts

@@ -126,6 +126,7 @@ export const useMessageStore = create<MessageState>((set, get) => ({
     if (state.isLoadingOlder || state.hasMoreByChannel[channelId] === false) return;
     const existing = state.messagesByChannel[channelId] || [];
     if (existing.length === 0) return;
+    // ponytail: existing is now oldest-first, so existing[0] is the true oldest
     const oldestId = existing[0].id;
     set({ isLoadingOlder: true });
     try {