refactor(format): hoist the relative-time tiers and lock their invariants

Review follow-ups on the tier-shift fix. The table moves to a module-level
RELATIVE_UNITS: it is constant, so rebuilding it per call was pure garbage on
a path that runs once per rendered timestamp across a virtualized feed.

Its ascending order is load-bearing and was unstated — sorting it largest-first
would make every timestamp under a year read "just now", silently, the same
shape as the bug this branch fixes. It is now documented and asserted, and the
suite derives its tier coverage and translation checks from the table itself
rather than from its own case list, so a new tier cannot slip in uncovered.

The audit log read the empty string for an unreadable date as a value and
rendered a blank cell; it now uses the `|| "—"` idiom the channel table
already uses, so missing and unreadable dates look alike.

frontend/src/lib/format.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
-import { relativeTime, relativeFromMs } from "./format";
+import { RELATIVE_UNITS, relativeTime, relativeFromMs } from "./format";
 import en from "../i18n/locales/en/time.json";
 import hu from "../i18n/locales/hu/time.json";
 
@@ -72,8 +72,27 @@ describe("relativeFromMs", () => {
     expect(relativeFromMs(NaN)).toBe("");
   });
 
-  it("translates every label these cases expect, in both languages", () => {
-    const keys = [...new Set(CASES.map(([, , label]) => label.split(":")[0].replace("time.", "")))];
+  // The three below are derived from RELATIVE_UNITS, not from CASES, so adding a tier to the
+  // table without covering it here fails rather than slipping through unnoticed.
+  it("keeps the unit table sorted ascending", () => {
+    const divisors = RELATIVE_UNITS.map(([secs]) => secs);
+    expect(divisors, "row 0 doubles as the just-now threshold").toEqual(
+      [...divisors].sort((a, b) => a - b),
+    );
+    expect(new Set(divisors).size, "duplicate divisors would make a tier unreachable").toBe(
+      divisors.length,
+    );
+  });
+
+  it("exercises every tier in the unit table", () => {
+    const covered = new Set(CASES.map(([, , label]) => label.split(":")[0]));
+    for (const [, key] of RELATIVE_UNITS) {
+      expect(covered, `no case covers ${key}`).toContain(key);
+    }
+  });
+
+  it("translates every label the table can emit, in both languages", () => {
+    const keys = ["justNow", ...RELATIVE_UNITS.map(([, key]) => key.replace("time.", ""))];
     for (const key of keys) {
       expect(en, `en/time.json is missing ${key}`).toHaveProperty(key);
       expect(hu, `hu/time.json is missing ${key}`).toHaveProperty(key);