fix: DM reactions + eradicate broken CSS variable opacity modifiers

Server-side: handleReactionAdd/Remove now check dm_messages table when
message isn't found in server messages, enabling DM emoji reactions
via the same unified client event path.

Client-side: replaced all remaining broken opacity modifiers on CSS
variables (border-border-soft/50, bg-accent-rose/10, bg-surface-*/N,
etc.) with explicit rgba values. Tailwind can't decompose var() for
opacity, causing fallback to currentColor. Fixes 16 files across
error boxes, table borders, surface tints, and input separators.

packages/web/src/components/chat/Message.tsx

@@ -286,7 +286,7 @@ export function Message({ message, isCompact, isFirstInGroup }: MessageProps) {
                       key={att.id}
                       href={`/api/uploads/${att.filename}`}
                       download={att.originalName}
-                      className="flex items-center gap-3 p-4 bg-surface-channel/50 rounded-lg border border-border-hard hover:bg-interactive-hover transition-all max-w-[400px] mt-1 group/att"
+                      className="flex items-center gap-3 p-4 bg-[rgba(26,26,35,0.50)] rounded-lg border border-border-hard hover:bg-interactive-hover transition-all max-w-[400px] mt-1 group/att"
                     >
                       <div className="p-2 bg-surface-base rounded text-txt-tertiary group-hover/att:text-txt-primary transition-colors">
                         <svg className="w-8 h-8" fill="none" viewBox="0 0 24 24" stroke="currentColor">