refactor(i18n): a description is one sentence, in all six languages

Measured before the sweep: 317 description and hint strings under the
`settings` and `admin` namespaces, median 83 characters, 113 of them two
sentences or more. The rule the design standards now carry is one sentence,
guideline 120 characters, and this is the sweep that makes it true.

Almost nothing was retranslated. A second sentence is already written in
every language — it is just in the wrong slot. `scripts/split-description.mjs`
cuts a description at its first sentence boundary in all six bundles at once
and either drops the tail or files it under its own key; the twenty-one keys
that moved now render in the card body as foreground `text-sm`, which is
where multi-sentence prose belongs and where a reader can actually read it
(the slot is muted `text-xs`). Eight tails were dropped outright, because
"Off by default" under a switch, "Topics and frequency are adjustable" above
the topic switches, and "Only admins can create invitations" on an
admin-gated route are not information.

Twenty-eight keys are gone: the privacy page's intro, which was a table of
contents for the four cards under it; two About descriptions that listed
verbatim the labels and the buttons beneath them; a card description that
repeated its own title; the API-token sentence that said what the paragraph
below it said with the actual instruction; and the eight integration
secondary sentences. Six descriptions were rewritten shorter in all six
languages where trimming was honest rather than lossy.

On the strings that changed: 4391 characters of English down to 2290, and 32
multi-sentence descriptions down to zero.

The rule is now a gate. `card-description-length.test.ts` reads the keys that
actually reach a card header's description slot — statically, from the JSX —
and checks them in EN and DE, because a rule that only holds in English
produces a slot that wraps to two lines in half the locales. Its exception
list is empty and carries a reason requirement plus a test that removes a
stale entry. Verified by putting a second sentence back: it fails.

scripts/split-description.mjs

@@ -0,0 +1,78 @@
+/**
+ * Splits an over-long description at its first sentence boundary, in every
+ * locale, WITHOUT retranslating anything: the tail is already written in each
+ * language, it is just in the wrong slot.
+ *
+ *   node scripts/split-description.mjs --dry <key> [<key> …]
+ *   node scripts/split-description.mjs drop <key> [<key> …]
+ *   node scripts/split-description.mjs move <key> <tailKey>
+ *
+ *   drop  — the description keeps sentence 1; the tail is deleted.
+ *   move  — the description keeps sentence 1; the tail lands under `tailKey`,
+ *           to be rendered in the card body as foreground `text-sm`.
+ *   --dry — print what each locale would become, change nothing.
+ *
+ * A terminator followed by whitespace and an upper-case letter (or a digit,
+ * or an opening quote) is a sentence boundary. `Claude.ai` and `z. B.` do not
+ * match: the first has no whitespace, the second continues lower-case.
+ */
+import { readFileSync, writeFileSync } from "node:fs";
+
+const LOCALES = ["de", "en", "es", "fr", "it", "pl"];
+const BOUNDARY = /(?<=[.!?])\s+(?=[A-ZÄÖÜÀÂÉÈÊÎÔÙÛÇŁŚŹŻĄĆĘŃÓ0-9„"„«])/u;
+
+function get(bundle, path) {
+  return path
+    .split(".")
+    .reduce((node, part) => (node == null ? node : node[part]), bundle);
+}
+function set(bundle, path, value) {
+  const parts = path.split(".");
+  const last = parts.pop();
+  let node = bundle;
+  for (const part of parts) {
+    if (typeof node[part] !== "object" || node[part] === null) node[part] = {};
+    node = node[part];
+  }
+  node[last] = value;
+}
+
+const [mode, ...rest] = process.argv.slice(2);
+const dry = mode === "--dry";
+const keys = dry ? rest : mode === "move" ? [rest[0]] : rest;
+const tailKey = mode === "move" ? rest[1] : null;
+if (!mode || keys.length === 0) {
+  console.error("usage: --dry|drop|move <key> [tailKey]");
+  process.exit(1);
+}
+
+for (const locale of LOCALES) {
+  const file = `messages/${locale}.json`;
+  const bundle = JSON.parse(readFileSync(file, "utf8"));
+  let changed = false;
+  for (const key of keys) {
+    const value = get(bundle, key);
+    if (typeof value !== "string") {
+      console.log(`${locale} ${key}: MISSING`);
+      continue;
+    }
+    const parts = value.split(BOUNDARY);
+    if (parts.length < 2) {
+      console.log(`${locale} ${key}: single sentence, untouched`);
+      continue;
+    }
+    const head = parts[0].trim();
+    const tail = parts.slice(1).join(" ").trim();
+    if (dry) {
+      console.log(`${locale} ${key}\n  head(${head.length}): ${head}\n  tail: ${tail}`);
+      continue;
+    }
+    set(bundle, key, head);
+    if (tailKey) set(bundle, tailKey, tail);
+    changed = true;
+  }
+  if (changed) {
+    writeFileSync(file, `${JSON.stringify(bundle, null, 2)}\n`, "utf8");
+    console.log(`${locale}: written`);
+  }
+}