Add timezone preference and fix all date handling to be timezone-aware

Dates were computed using browser/server local time with no explicit timezone,
causing inconsistencies when server runs in UTC. Now all "today" computations
and date formatting use the user's chosen IANA timezone, persisted in
localStorage and selectable from Settings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/format.ts

@@ -1,4 +1,9 @@
-// fmt.* — verbatim port from primitives.jsx
+import { getToday } from "./tz.js";
+
+function parseDate(s: string): Date {
+  return new Date(s.length === 10 ? s + "T12:00:00" : s);
+}
+
 export const fmt = {
   g(n: number | null | undefined): string {
     if (n == null) return "—";
@@ -17,22 +22,26 @@ export const fmt = {
     if (n == null) return "—";
     return `${(+n).toFixed(1)}%`;
   },
-  date(s: string | null | undefined): string {
+  date(s: string | null | undefined, tz?: string): string {
     if (!s) return "—";
-    const d = new Date(s);
-    return d.toLocaleDateString("en-US", { month: "short", day: "numeric", year: "numeric" });
+    const opts: Intl.DateTimeFormatOptions = { month: "short", day: "numeric", year: "numeric" };
+    if (tz) opts.timeZone = tz;
+    return parseDate(s).toLocaleDateString("en-US", opts);
   },
-  dateShort(s: string | null | undefined): string {
+  dateShort(s: string | null | undefined, tz?: string): string {
     if (!s) return "—";
-    const d = new Date(s);
-    return d.toLocaleDateString("en-US", { month: "short", day: "numeric" });
+    const opts: Intl.DateTimeFormatOptions = { month: "short", day: "numeric" };
+    if (tz) opts.timeZone = tz;
+    return parseDate(s).toLocaleDateString("en-US", opts);
   },
-  daysAgo(s: string | null | undefined): string {
+  daysAgo(s: string | null | undefined, tz?: string): string {
     if (!s) return "—";
-    const ms = Date.now() - new Date(s).getTime();
-    const d = Math.floor(ms / 86_400_000);
+    const todayMs = +parseDate(tz ? getToday(tz) : new Date().toISOString().slice(0, 10));
+    const thenMs = +parseDate(s);
+    const d = Math.floor((todayMs - thenMs) / 86_400_000);
     if (d === 0) return "today";
     if (d === 1) return "yesterday";
+    if (d < 0) return "in the future";
     if (d < 30) return `${d}d ago`;
     if (d < 365) return `${Math.floor(d / 30)}mo ago`;
     return `${Math.floor(d / 365)}y ago`;