feat(web): discord-style chat, members pane, invitations and optimistic send [CI]

- chat: left-aligned discord-style messages with grouped senders, hover
  actions, an auto-growing composer and file cards
- send: render messages and files locally first and replace the
  placeholder when the worker publishes the real item; failures keep the
  text with a retry action
- navigation: space rail plus a flatter channel list with the space name
  header and collapsible groups
- members: right pane tabs (files / members) grouped by online, offline
  and pending users, with a prominent invite action and a top bar shortcut
- notifications: toast host for invite results; invitees get a toast with
  an accept action, and the invitation payload now carries the space name
  and inviter inside the encrypted grant
- users over devices: drop device names, the device list and device
  rename/release; senders show the account (guests show as guest) and the
  share panel only keeps links
- follow contract 7.0.0 (max_members, members_used, members_full, /join)
- docs: ui-design and frontend updated for the new layout and concepts

web/src/features/channel/Workspace.module.css

@@ -1,8 +1,9 @@
 .desktop {
-  --nav-width: 320px;
+  --rail-width: 64px;
+  --nav-width: 240px;
 
   display: grid;
-  grid-template-columns: var(--nav-width) minmax(260px, 1fr) 6px clamp(320px, var(--files-width, 36%), calc(100% - var(--nav-width) - 266px));
+  grid-template-columns: var(--rail-width) var(--nav-width) minmax(260px, 1fr) 6px clamp(320px, var(--files-width, 36%), calc(100% - var(--rail-width) - var(--nav-width) - 266px));
   grid-template-rows: auto minmax(0, 1fr);
   height: 100vh;
   height: 100dvh;
@@ -13,6 +14,7 @@
   border-bottom: 1px solid var(--line);
 }
 
+.rail,
 .navigation,
 .center,
 .files,
@@ -23,6 +25,11 @@
   min-height: 0;
 }
 
+.rail {
+  overflow-y: auto;
+  background: var(--bg);
+}
+
 .navigation {
   overflow-y: auto;
   border-right: 1px solid var(--line);
@@ -125,6 +132,6 @@
 
 @media (width >= 900px) and (width <= 1100px) {
   .desktop {
-    --nav-width: 272px;
+    --nav-width: 220px;
   }
 }