Offline reverse-geocoding for GPS to city

.gitignore

@@ -144,3 +144,6 @@ vite.config.ts.timestamp-*
 
 # Turborepo
 .turbo
+
+# Local EXIF test images (downloaded, not part of the repo)
+/test-assets/

apps/web/package.json

@@ -19,6 +19,7 @@
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "lucide-react": "^1.21.0",
+    "offline-geocode-city": "^1.0.2",
     "react": "^19.2.7",
     "react-dom": "^19.2.7",
     "react-dropzone": "^15.0.0",

apps/web/src/lib/geocode.ts

@@ -0,0 +1,33 @@
+export interface PlaceName {
+  city: string
+  country: string
+  countryIso2: string
+}
+
+// The cities dataset (~1.7 MB) is lazy-loaded on first use so it never bloats
+// the initial bundle. Everything runs locally — coordinates never leave the
+// device (see docs/decisions/0001-offline-reverse-geocoding.md).
+let loader: Promise<typeof import('offline-geocode-city')> | null = null
+
+function loadGeocoder() {
+  loader ??= import('offline-geocode-city')
+  return loader
+}
+
+export async function reverseGeocode(
+  latitude: number,
+  longitude: number,
+): Promise<PlaceName | null> {
+  try {
+    const { getNearestCity } = await loadGeocoder()
+    const result = await getNearestCity(latitude, longitude)
+    if (!result?.cityName) return null
+    return {
+      city: result.cityName,
+      country: result.countryName,
+      countryIso2: result.countryIso2,
+    }
+  } catch {
+    return null
+  }
+}

docs/decisions/0001-offline-reverse-geocoding.md

@@ -0,0 +1,33 @@
+# ADR 0001 — Offline reverse-geocoding for EXIF captions
+
+**Status:** Accepted (2026-06-28)
+
+## Context
+
+Polaroid captions auto-fill the city from a photo's EXIF GPS (#4). Resolving
+GPS → city name can be done two ways:
+
+- **Online API** (Nominatim, Google, etc.) — accurate, but sends every photo's
+  coordinates to a third party.
+- **Offline dataset** — bundled cities, nearest-neighbour lookup, fully local.
+
+The app's core promise is _"private, client-side, never uploaded"_. Sending
+coordinates to a geocoding API silently breaks that promise.
+
+## Decision
+
+**Offline-first.** Use
+[`offline-geocode-city`](https://www.npmjs.com/package/offline-geocode-city) — a
+bundled cities dataset (~1.7 MB) exposing `getNearestCity(lat, lon)` →
+`{ cityName, countryName, countryIso2 }`. It runs entirely in the browser;
+coordinates never leave the device.
+
+To keep the initial bundle small, the dataset is **lazy-loaded** (dynamic
+`import()`) only when the first GPS-tagged photo needs geocoding.
+
+## Consequences
+
+- City accuracy is "nearest sizable city", not street-level — fine for captions.
+- No network, no API keys, no rate limits, no privacy leak.
+- If higher accuracy is ever wanted, an **opt-in** online lookup (off by
+  default, clearly disclosed) can be added later — never silent.