Show device timestamps in the reader's local time

The exact issuance and expiry times in the device rows' hover titles were
rendered in UTC, which reads as wrong to anyone not living there.

- Mark the dates with a <time> element carrying the absolute (UTC) timestamp,
  rather than a custom data attribute whose name claimed to hold local time
- Rewrite those titles in the browser's own zone, keeping the server's UTC
  rendering as the fallback for when JavaScript is unavailable
- Only convert values that carry an explicit offset, so a date-only <time> is
  not read as UTC midnight and shifted onto the previous day
- Give the revoke button a little more room from the device name and dates

Part of #254

app/javascript/local_time.js

@@ -0,0 +1,32 @@
+// Timestamps are rendered by the server in UTC, next to a <time> element's
+// machine-readable value. Rewrite those elements' hover titles so they show the
+// reader's own local time instead.
+
+const LOCAL_TIME_FORMAT = new Intl.DateTimeFormat("en-GB", {
+  day: "numeric",
+  month: "short",
+  year: "numeric",
+  hour: "2-digit",
+  minute: "2-digit",
+  hourCycle: "h23",
+})
+
+// Only absolute timestamps carry a zone to convert from: a date-only value would
+// be read as UTC midnight and shift to the previous day for most readers.
+const ABSOLUTE_TIMESTAMP = /T\d{2}:\d{2}(:\d{2})?(\.\d+)?(Z|[+-]\d{2}:\d{2})$/
+
+const localizeTimestamps = () => {
+  document.querySelectorAll("time[datetime]").forEach((element) => {
+    const value = element.getAttribute("datetime")
+
+    if (!ABSOLUTE_TIMESTAMP.test(value)) return
+
+    const timestamp = new Date(value)
+
+    if (!Number.isNaN(timestamp.getTime())) {
+      element.title = LOCAL_TIME_FORMAT.format(timestamp)
+    }
+  })
+}
+
+document.addEventListener("turbo:load", localizeTimestamps)