feat(web): split the home page into quick share and spaces zones

Two side-by-side zones (stacked on narrow screens) with an icon and a
one-line positioning each: quick share is temporary, read-only and needs
no account; spaces are long-lived, multi-user and need a login. Shorter
copy, my shares listed under the share input, and an empty state for
spaces.

web/src/features/home/QuickShare.module.css

@@ -14,36 +14,6 @@
   background: var(--busy-soft);
 }
 
-.head {
-  display: flex;
-  align-items: flex-start;
-  justify-content: space-between;
-  gap: var(--space-3);
-}
-
-.title {
-  margin: 0;
-  font-size: 18px;
-}
-
-.hint {
-  margin: var(--space-1) 0 0;
-  color: var(--text-dim);
-  font-size: 13px;
-}
-
-.tag {
-  display: inline-flex;
-  flex: 0 0 auto;
-  align-items: center;
-  gap: var(--space-1);
-  padding: var(--space-1) var(--space-2);
-  border-radius: var(--radius-sm);
-  background: var(--ok-soft);
-  color: var(--ok);
-  font-size: 12px;
-}
-
 .editor {
   width: 100%;
   min-height: 112px;