feat: dynamic PiP collision avoidance, reaction pill redesign, channel dedup fix

PiP positioning:
- Replace hardcoded layout constants with dynamic DOM measurement system
- Obstacle elements declare themselves via data-pip-obstacle="left|bottom"
- getPipBounds() queries actual element rects at runtime via getBoundingClientRect
- MutationObserver + ResizeObserver re-clamp PiP when obstacles appear/resize
- PiP reappears after close when navigating away from voice channel

Reactions:
- Align reaction pills with Aether Drift prototype
- Own-reaction accent: purple → mint (bg, border, count color)
- Default pills: subtle white overlay bg + border-soft border
- Count text: 12px/semibold/txt-secondary per prototype spec

Bug fix:
- Deduplicate channel insertion in createChannel store action
- Prevents double-add race between REST response and WS broadcast

packages/web/src/components/layout/ServerSidebar.tsx

@@ -157,7 +157,7 @@ export function ServerSidebar() {
   };
 
   return (
-    <nav className="w-[72px] bg-surface-base flex flex-col items-center py-3 overflow-y-auto flex-shrink-0 no-scrollbar select-none md:fixed md:inset-y-0 md:left-0 md:z-[100] md:glass-strip">
+    <nav data-pip-obstacle="left" className="w-[72px] bg-surface-base flex flex-col items-center py-3 overflow-y-auto flex-shrink-0 no-scrollbar select-none md:fixed md:inset-y-0 md:left-0 md:z-[100] md:glass-strip">
       <SidebarItem
         id="@me"
         name="Direct Messages"