v0.6.2: show and manage on-phone sessions offline and signed out

A city-map timer started without signing in did not appear under Sessions.

The tab returned "Sign in to see your sessions" before rendering anything, so
a local session could never show; and even signed in it only ever listed
ParkSmarter's sessions. A local timer is the one kind that has no server copy,
which made it the one kind the screen could not display.

- Sessions now leads with "Tracking on this phone": the live countdown with
  +1 hour and End, working with no account and no network, because that is the
  only place the session exists.
- Recent local sessions are kept in a small on-device history (50 max). Without
  it a local session vanished the instant it ended — there is no server to ask.
  Paid ParkSmarter sessions are excluded so they don't appear twice.
- The ParkSmarter half is layered on top when signed in and can fail
  independently: offline it reports that and keeps the local half visible,
  rather than the whole tab going blank. It also no longer leaves an unhandled
  rejection when the fetch throws (it had try/finally but no catch).
- The card's second button follows the notification's rule: +1 hour for a local
  timer, Extend -> purchase screen for a bought session, since only one of those
  can honestly add time.

History is written in endActiveParking() before the record is dropped, which
also covers expiry — syncActiveParking() routes a lapsed session through the
same call.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -73,6 +73,11 @@ offer to run a 4-hour timer — that's just scheduling a ticket). The ongoing no
 second button reads **+1 hr** here rather than *Extend*: there is nothing to buy, so it
 edits the local timer and says so.
 
+The **Sessions** tab shows and manages these under *Tracking on this phone* — add an hour,
+end it, and see recent ones — with no account and no network, because that is the only
+place they exist. ParkSmarter's own sessions are layered on top when you're signed in, and
+failing to reach them (offline, or signed out) never hides the local half.
+
 The georeference was fitted to OpenStreetMap street centrelines and lands within ~4 m
 (see [`tools/citymap/`](tools/citymap/) to regenerate it from a new edition of the PDF).
 Because a few metres is the difference between two sides of a street, **Account → Align city