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)

internal/db/db.go

@@ -277,6 +277,15 @@ CREATE TABLE IF NOT EXISTS conversation_messages (
 CREATE INDEX IF NOT EXISTS idx_conversation_members_user ON conversation_members(user_id);
 CREATE INDEX IF NOT EXISTS idx_conversation_messages_conv_created ON conversation_messages(conversation_id, created_at DESC);
 
+CREATE TABLE IF NOT EXISTS conversation_reactions (
+	id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+	message_id UUID NOT NULL REFERENCES conversation_messages(id) ON DELETE CASCADE,
+	user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+	emoji VARCHAR(64) NOT NULL,
+	created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+	UNIQUE(message_id, user_id, emoji)
+);
+
 -- Moderation
 CREATE TABLE IF NOT EXISTS bans (
 	id UUID PRIMARY KEY DEFAULT gen_random_uuid(),