fix: cross-machine access + center filter bar + floating hints

- api.ts: switch baseURL from http://localhost:8001/api/v1 to relative
  /api/v1. Both nginx (prod) and vite (dev) already proxy /api/ to the
  backend, so requests become same-origin and the app works from any
  host (LAN IP, reverse proxy, another machine) with no CORS dance.
- backend CORS: open to "*" as a fallback for the rare direct-hit case;
  the normal flow is same-origin via the proxy and never touches CORS.
- App layout: move FilterBar and DiscardActionBar inside the main
  content column (right of the left sidebar) so the filter row no
  longer bleeds across the sidebar.
- FilterBar: justify-center the pills so they sit centered above the
  timeline. Clear-all uses ml-2 instead of ml-auto.
- KeyboardHints: convert to a floating, glassy pill pinned bottom-
  center (fixed positioning + backdrop-blur + ring) instead of a flat
  toolbar row. Removed from the column layout — now mounted as an
  overlay sibling.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/KeyboardHints.tsx

@@ -25,22 +25,22 @@ export function KeyboardHints() {
       ]
 
   return (
-    <div className="flex justify-center border-b border-border bg-surface/60 px-4 py-1.5">
-      <div className="flex items-center gap-3">
+    <div className="pointer-events-none fixed bottom-4 left-1/2 z-30 -translate-x-1/2">
+      <div className="pointer-events-auto flex items-center gap-3 rounded-full border border-border/60 bg-surface/40 px-4 py-1.5 shadow-lg ring-1 ring-white/5 backdrop-blur-md">
         {hints.map((hint, i) => (
           <div key={i} className="flex items-center gap-1.5">
-            <kbd className="rounded bg-surface-offset px-2 py-0.5 text-[11px] font-medium text-text">
+            <kbd className="rounded bg-surface-offset/80 px-1.5 py-0.5 text-[11px] font-medium text-text">
               {hint.key}
             </kbd>
             <span className="text-xs text-text-muted">{hint.action}</span>
             {i < hints.length - 1 && (
-              <span className="ml-2 text-text-faint">•</span>
+              <span className="ml-1 text-text-faint">•</span>
             )}
           </div>
         ))}
         {selectedCount > 0 && (
           <>
-            <span className="ml-2 text-text-faint">•</span>
+            <span className="text-text-faint">•</span>
             <span className="text-xs font-medium text-primary">
               {selectedCount} selected
             </span>