refactor(format): localize relative time with Intl.RelativeTimeFormat

The tier table kept a divisor and a translation key as two independently
maintained halves, and a mispairing there rendered minutes as "Ns ago" for 59
releases. The table now pairs a divisor with the Intl unit it produces and the
platform supplies the words, so the wrong half is at least self-evident on
sight — "minute" next to 3600 reads wrong in a way "time.minutesAgo" never
did. It also removes 12 hand-written strings and hands plural rules to CLDR
for any language added later.

Hungarian output is byte-identical to the strings it replaces. English gets
shorter: "59 min ago" to "59m ago", "1 wk ago" to "1w ago", "1 mo ago" to
"1mo ago", "1 yr ago" to "1y ago"; "2h ago" and "1d ago" are unchanged.

style narrow matches the app's compact timestamps and numeric always keeps it
counting, rather than "yesterday"/"last week". time.justNow stays because Intl
has no wording for the sub-minute case. Formatters are memoized per language,
since this runs once per rendered timestamp across a virtualized feed.

frontend/src/i18n/locales/en/time.json

@@ -1,11 +1,5 @@
 {
   "justNow": "just now",
-  "minutesAgo": "{{count}} min ago",
-  "hoursAgo": "{{count}}h ago",
-  "daysAgo": "{{count}}d ago",
-  "weeksAgo": "{{count}} wk ago",
-  "monthsAgo": "{{count}} mo ago",
-  "yearsAgo": "{{count}} yr ago",
   "eta": {
     "now": "now",
     "done": "done",

frontend/src/i18n/locales/hu/time.json

@@ -1,11 +1,5 @@
 {
   "justNow": "épp most",
-  "minutesAgo": "{{count}} perce",
-  "hoursAgo": "{{count}} órája",
-  "daysAgo": "{{count}} napja",
-  "weeksAgo": "{{count}} hete",
-  "monthsAgo": "{{count}} hónapja",
-  "yearsAgo": "{{count}} éve",
   "eta": {
     "now": "most",
     "done": "kész",