Add share target, prompt templates, command palette and session organisation

- PWA share target: shared text/links start a new chat or go into a
  recent session (chooser dialog)
- Prompt templates (personal + global for admins) with {{placeholders}}
  that are asked for on insert; managed under Settings → Templates
- Ctrl/Cmd+K command palette; Alt+N new session, Alt+Up/Down switch
  sessions, Ctrl+, settings
- Sessions can be pinned, archived and filed into folders; the list can
  be grouped by date or by project, groups are collapsible
- Fix: the "What's new" dialog was never shown to signed-in users

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/components/Composer.tsx

@@ -1,3 +1,4 @@
+import { TemplatePicker } from "./TemplatePicker";
 import { Icon } from "./Icon";
 import { formatSize } from "./FileFigure";
 import { uploadFile } from "../api";
@@ -65,6 +66,7 @@ export function Composer({
   // Reasoning effort for the next message only ("" = model default).
   const [effort, setEffort] = useState("");
   const [interim, setInterim] = useState("");
+  const [picker, setPicker] = useState(false);
   const models = useModels();
   const [prov, ...mid] = session.model.split("/");
   const reasoning = !!models.find((p) => p.provider === prov)?.models.find((m) => m.id === mid.join("/"))?.reasoning;
@@ -322,6 +324,19 @@ export function Composer({
           </select>
           <ModelSelect value={session.model} onChange={(m) => onUpdate({ model: m })} title={t("composer.model")} />
           {reasoning && <EffortSelect value={effort} onChange={setEffort} compact />}
+          <button className="ghost" style={{ padding: "3px 6px" }} onClick={() => setPicker(true)} title={t("templates.title")}>
+            <Icon name="template" />
+          </button>
+          {picker && (
+            <TemplatePicker
+              onClose={() => setPicker(false)}
+              onPick={(x) => {
+                setPicker(false);
+                setText(text.trim() ? `${text.replace(/\s+$/, "")}\n\n${x}` : x);
+                setTimeout(() => ref.current?.focus(), 0);
+              }}
+            />
+          )}
           <VoiceButton onText={(d) => setText(text ? `${text.replace(/\s+$/, "")} ${d}` : d)} onInterim={setInterim} />
           <label className="ghost" style={{ cursor: "pointer", padding: "3px 6px" }} title={t("composer.attach")}>
             <Icon name="clip" />