Build the two rule descriptions as sentences, not fragments

Both describeRule functions assembled their output by concatenation, which no
catalogue could fix. A translator handed " and " or " on " in isolation cannot
move it: German puts the verb last, Japanese does not separate list items with
a word at all, and the fragments arrive in an order the English sentence chose.
Reported by a native speaker reviewing the German catalogue (#247), whose "the
summaries" item is the Sieve one.

Every branch is now one whole sentence with placeholders, so a translator
rewrites the sentence including its word order. Joining is Intl.ListFormat,
which gives "A, B und C" for an allof rule and the language's own disjunction
for anyof, rather than a hardcoded " and " that would be wrong twice over.

The recurrence tail no longer appends: ", 5 times" and ", until 2026-05-03"
wrap the sentence they qualify, so a language that puts the limit first can.

Ordinals become words. The old suffix table -- st, nd, rd, th, picked by
arithmetic -- is English spelling rules in code, and no catalogue can reach a
suffix chosen that way. German writes "1.", Japanese "第1". nthOfPeriod is 1-5
or -1 in practice, so five words and "last" cover it.

WEEKDAYS is gone. Its long names could have been catalogue entries but its
short ones never could: "T" is Tuesday and Thursday, "S" is Saturday and
Sunday, and a catalogue cannot hold two translations under one key. That was
bad data rather than missing translation, and Intl has every name in every
locale in three widths. lib/datetime.ts gains weekdayName, weekdayNames and
formatList; recurrence.ts keeps WEEKDAY_KEYS for the ordering, which is not a
language question.

Adds the first tests either function has had. Neither had any, and no test
would have caught what was wrong with them, since the English output was
correct -- so these pin the two properties that actually matter: fragments go
through the catalogue, and the joining is Intl's.

32 strings and 9 plural forms are new and land with each language.

Verified: typecheck clean, 1009 tests pass.

web/src/views/calendar/EventEditor.tsx

@@ -13,7 +13,7 @@ import { RecipientInput } from "../compose/RecipientInput";
 import { DateField, DateTimeField } from "@/ui/datefield";
 import { browserTimeZone, dateToZonedLocal, formatDuration, fromInputDateTime, listTimeZones, parseDuration, toInputDateTime, toLocalDateOnly, zonedToDate, DAY_MS, humanDuration } from "@/lib/dates";
 import { formatClock, formatNumericDate, formatWeekday, formatWeekdayDate } from "@/lib/datetime";
-import { WEEKDAYS, describeRule, presetFor, ruleFromPreset, type RecurrencePreset } from "@/lib/recurrence";
+import { WEEKDAY_KEYS, weekdayOptions, describeRule, presetFor, ruleFromPreset, type RecurrencePreset } from "@/lib/recurrence";
 import { newKey } from "@/lib/contacts";
 import { availabilityWindow } from "@/lib/availabilityWindow";
 import { askEditScope, droppedMessage, runScoped } from "./scope";
@@ -359,7 +359,7 @@ function EventForm({ init, base, scope, editing, onClose, settingsTz, defaultAle
             </select>
           )}
           {!oneDate && (
-          <select className="select" style={{ width: "auto", height: 32 }} value={preset} onChange={(e) => { const p = e.target.value as RecurrencePreset; setPreset(p); if (p === "custom") setRule(rule ?? { "@type": "RecurrenceRule", frequency: "weekly", byDay: [{ "@type": "NDay", day: WEEKDAYS[(start.getDay() + 6) % 7]!.key }] }); else setRule(ruleFromPreset(p, start)); }}>
+          <select className="select" style={{ width: "auto", height: 32 }} value={preset} onChange={(e) => { const p = e.target.value as RecurrencePreset; setPreset(p); if (p === "custom") setRule(rule ?? { "@type": "RecurrenceRule", frequency: "weekly", byDay: [{ "@type": "NDay", day: WEEKDAY_KEYS[(start.getDay() + 6) % 7]! }] }); else setRule(ruleFromPreset(p, start)); }}>
             <option value="none">{translate("Does not repeat")}</option>
             <option value="daily">{translate("Daily")}</option>
             <option value="weekly">{translate("Weekly on {weekday}", { weekday: formatWeekday(start, "long") })}</option>
@@ -381,7 +381,7 @@ function EventForm({ init, base, scope, editing, onClose, settingsTz, defaultAle
             </div>
             {customRule.frequency === "weekly" && (
               <div className="row" style={{ gap: 4, marginTop: 8 }}>
-                {WEEKDAYS.map((w) => {
+                {weekdayOptions().map((w) => {
                   const on = customRule.byDay?.some((d) => d.day === w.key);
                   return <button key={w.key} type="button" className={`btn btn-sm btn-pill ${on ? "btn-primary" : ""}`} style={{ width: 36, padding: 0 }} title={w.label} onClick={() => { const cur = customRule.byDay ?? []; const next: JSCalendarNDay[] = on ? cur.filter((d) => d.day !== w.key) : [...cur, { "@type": "NDay", day: w.key }]; setRule({ ...customRule, byDay: next.length ? next : undefined }); }}>{w.short}</button>;
                 })}