Animate the radar timeline with a double-buffered overlay

Completes milestone 3: frames render over the map, play as a loop, and can be
scrubbed, with the legend drawn from the manifest rather than a constant that
could drift from what the worker actually rendered.

The overlay alternates two MapLibre image sources. Updating one source in place
flickers, because the layer briefly shows a half-written texture; adding every
frame as its own layer avoids that but pins them all in GPU memory, and twenty
512x512 RGBA frames is about 80 MB. Two buffers cost the same whether the
timeline holds six frames or sixty. Platform-channel work is serialised because
`show` is called faster than the round trip completes during playback, and
overlapping updates would swap visibility out of order and strobe.

Prefetching loads a window around the playhead, nearest first and forward
before backward, since playback moves forward and that frame is needed
soonest. `FrameCache` is byte-budgeted rather than entry-counted because frame
size tracks how much precipitation is on screen, and it evicts by distance from
the playhead: plain LRU would keep frames the prefetcher touched a moment ago
even after the playhead moved to the far end of the timeline.

Also adds DpcRadarSource, which reads published frames from our CDN and never
from the DPC API. Without it, the `dpc` adapter would have had to fall back to
mock, putting demo frames on screen under the label of live data — exactly the
confusion the adapter split exists to prevent. It now fails naming the missing
setting instead.

Running it on the emulator caught three things the tests had not:

- The notifier wrote to `state` from inside `build()`, which Riverpod rejects
  as an uninitialised provider. That broke startup, not just tests.
- Eight-month-old demo frames rendered as "Aggiornato 342535 minuti fa". The
  age formatter now steps up to hours and days.
- Demo mode sat permanently behind a stale-data warning and so never showed the
  working state it exists to demonstrate. MockRadarSource now shifts the
  bundled timestamps onto the present, leaving images, order and spacing
  untouched, so the timeline behaves exactly as it would on live data.

Corrects docs/stack-decisions.md, which described a disk cache that was not
built: mock frames already live in the asset bundle, so a disk layer belongs
with the network adapter where it would save a real request.

Verified: analyze clean, 129 tests passing, and on the emulator the loop
advances, wraps, and reports "Aggiornato ora".

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

Nuvolari/app/lib/l10n/app_it.arb

@@ -99,5 +99,61 @@
   "openLink": "Apri",
   "@openLink": {
     "description": "Accessibility label for a button that opens a link in the browser"
+  },
+  "timelinePlay": "Riproduci",
+  "@timelinePlay": {
+    "description": "Accessibility label and tooltip for the play button on the radar timeline"
+  },
+  "timelinePause": "Pausa",
+  "@timelinePause": {
+    "description": "Accessibility label and tooltip for the pause button on the radar timeline"
+  },
+  "timelineScrubber": "Scorri i fotogrammi radar",
+  "@timelineScrubber": {
+    "description": "Accessibility label for the timeline slider"
+  },
+  "radarNoData": "Nessun dato radar disponibile",
+  "@radarNoData": {
+    "description": "Shown when no radar manifest could be loaded and there is no cached one either"
+  },
+  "radarStaleWithAge": "Dati non aggiornati · {age}",
+  "@radarStaleWithAge": {
+    "description": "Banner shown when the newest radar frame is older than expected. The age is the already-formatted string from dataAgeMinutes.",
+    "placeholders": {
+      "age": {
+        "type": "String",
+        "example": "Aggiornato 25 minuti fa"
+      }
+    }
+  },
+  "radarLegendUnit": "Intensità ({unit})",
+  "@radarLegendUnit": {
+    "description": "Header of the radar colour legend, with the physical unit taken from the manifest",
+    "placeholders": {
+      "unit": {
+        "type": "String",
+        "example": "dBZ"
+      }
+    }
+  },
+  "dataAgeHours": "{hours, plural, one{Aggiornato 1 ora fa} other{Aggiornato {hours} ore fa}}",
+  "@dataAgeHours": {
+    "description": "Data age when it is measured in hours rather than minutes",
+    "placeholders": {
+      "hours": {
+        "type": "int",
+        "example": "3"
+      }
+    }
+  },
+  "dataAgeDays": "{days, plural, one{Aggiornato 1 giorno fa} other{Aggiornato {days} giorni fa}}",
+  "@dataAgeDays": {
+    "description": "Data age when it is measured in days. Radar this old is useless, but stating it plainly beats an unreadable minute count.",
+    "placeholders": {
+      "days": {
+        "type": "int",
+        "example": "4"
+      }
+    }
   }
 }