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/lib/slashCommands.ts

@@ -35,6 +35,7 @@ export const SLASH_COMMANDS: SlashCommand[] = [
   { name: 'me', description: 'Send an action message', transform: (a, u) => '*' + u + ' ' + a + '*' },
   { name: 'spoiler', description: 'Hide text as a spoiler', transform: (a) => '||' + a + '||' },
   { name: 'poll', description: 'Create a poll', transform: (a) => a },
+  { name: 'emoji', description: 'Open emoji picker', transform: (a) => a },
 ];
 
 export function findCommand(name: string): SlashCommand | undefined {