v0.6.4: refresh the map's GPS every 30 s while it's on screen

A single fix at mount went stale the moment you walked a block, which is
most of the time you'd be looking at the map.

useLocation takes { intervalMs, active }: it polls only while active,
reads immediately on becoming active (re-opening the tab is when the
held fix is most stale), asks for permission once rather than every
poll, and stops polling on a hard denial. The last-known-position
shortcut now runs only before there is anything to show — on a later
poll it is usually older than what we hold, so it would make the dot
jump backwards. It also returns updatedAt.

MapScreen gates the poll on screen focus AND app foreground; focus
alone isn't enough, since a backgrounded app stays "focused" on its
last tab and Android won't hand location to one that isn't in front.

"Park here" / "My location" took nativeFix.current ?? coords, so the
MapLibre dot's cached fix won unconditionally and never expired — a
fresher polled fix could never get past it. Both now use bestFix(),
which takes whichever source is under 45 s old (a poll and a half, so
an on-schedule fix never reads as stale), forces a read if neither is,
and falls back to a stale fix rather than nothing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017DE83epHK8WbkppNszG3A2

app/app.json

@@ -3,14 +3,14 @@
     "name": "BigBrainParking",
     "slug": "bigbrainparking",
     "scheme": "bigbrainparking",
-    "version": "0.6.3",
+    "version": "0.6.4",
     "orientation": "portrait",
     "userInterfaceStyle": "automatic",
     "newArchEnabled": true,
     "icon": "./assets/icon.png",
     "android": {
       "package": "top.mowden.bigbrainparking",
-      "versionCode": 24,
+      "versionCode": 25,
       "edgeToEdgeEnabled": true,
       "adaptiveIcon": {
         "foregroundImage": "./assets/adaptive-icon.png",