docs: add workspace collaboration design and client demos

Document shared workspaces, realtime messaging, member handling, and source permissions. Include local interaction demos and verification scripts, and update development startup for Electron Node mode. Production collaboration services remain unimplemented.

docs/demo/client/workspace.css

@@ -0,0 +1 @@
+.workspace-shell{grid-template-columns:235px minmax(0,1fr)}.workspace-shell main{min-width:0}.space-nav{display:grid;gap:7px}.space-nav button{text-align:left;border:0;background:transparent}.space-nav button.active{background:#dfe9fc;color:#285fc7}.workspace-content{max-width:1250px}.workspace-layout{display:grid;grid-template-columns:minmax(0,1fr) 280px;gap:22px}.workspace-feed{min-width:0}.thread{border:1px solid var(--line);background:white;border-radius:14px;margin-bottom:16px;padding:22px}.thread.selected{border-color:#9bb7ed}.thread-head{display:flex;align-items:center;justify-content:space-between;gap:12px;margin-bottom:12px}.thread-body{white-space:pre-wrap;overflow-wrap:anywhere;line-height:1.8}.thread-context{font-size:12px;color:var(--muted);padding:12px 0}.thread-reply{border-left:2px solid #b6cbf3;padding:8px 0 8px 16px;margin:12px 0}.thread-reply p{white-space:pre-wrap;color:#40516a;overflow-wrap:anywhere}.thread-actions{display:flex;gap:8px;flex-wrap:wrap;margin-top:16px}.workspace-composer textarea{min-height:110px}.recipient{display:flex;align-items:center;gap:10px;flex-wrap:wrap;margin:12px 0}.recipient .badge{font-size:12px}.mention-list{border:1px solid var(--line);padding:8px;border-radius:10px;background:#fff;display:grid;gap:4px;margin-bottom:12px}.mention-list button{text-align:left;border:0;display:flex;justify-content:space-between}.context-options{display:grid;gap:12px;margin:16px 0}.workspace-members{display:grid;gap:16px;margin-top:20px}.workspace-member{display:flex;gap:10px;align-items:center}.workspace-member strong{display:block;margin-bottom:4px}.workspace-modal{border:1px solid var(--line);border-radius:16px;padding:26px;width:min(480px,calc(100vw - 32px));color:inherit}.workspace-modal::backdrop{background:#14283d60}.workspace-modal h2{margin-bottom:18px}.workspace-toolbar{display:flex;gap:10px;align-items:center;flex-wrap:wrap}.workspace-toolbar select{width:auto}.context-preview{padding:12px;background:#f4f7fb;border-radius:8px;font-size:12px;white-space:pre-wrap;line-height:1.8}.workspace-notice{margin-bottom:20px}.workspace-status{min-height:20px;font-size:12px;color:#3569dc}.preference-row{display:grid;grid-template-columns:1fr 180px;gap:12px;align-items:center;padding:14px 0;border-top:1px solid var(--line)}.workspace-mobile-nav{display:none}.workspace-preferences{margin-top:24px}.thread details{margin-bottom:10px}@media(max-width:1050px){.workspace-layout{grid-template-columns:1fr}.workspace-layout>.aside{grid-template-columns:1fr 1fr}}@media(max-width:720px){.workspace-shell{display:block}.workspace-mobile-nav{display:flex;gap:8px;margin-bottom:18px}.workspace-mobile-nav select{min-width:0}.workspace-layout>.aside{grid-template-columns:1fr}.thread{padding:17px}.thread-head{align-items:flex-start}.preference-row{grid-template-columns:1fr}.workspace-toolbar{width:100%}.workspace-toolbar select{flex:1;min-width:0}}