v0.2.6: fix kiosk-code scan resolution + add zone QR asset

ScanScreen.lookupCode tried getMetersByScannerCode first and let its thrown
error-envelope abort the whole lookup, so the getMetersByZoneName fallback never
ran; it also sent ZoneName in the scanned case (the API is case-sensitive — the
official app lowercases). Confirmed via live mitmproxy capture of the official
app: a scanned code resolves through ScannerCode, and ZoneName=dl (lowercase)
returns the zone.

Now tries ScannerCode -> ZoneName -> zonename(lowercased) -> TerminalSerNo, each
independent and non-fatal; only reports "Lookup failed" if every attempt errored.

Adds docs/zone-qr/ with the DL-Sandpoint QR (encodes ScannerCode "DL", works in
both BigBrainParking and the official app) and a how-to.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/zone-qr/README.md

@@ -0,0 +1,36 @@
+# Zone QR codes
+
+A parking-zone QR code just encodes the zone's **`ScannerCode`** as plain text.
+BigBrainParking's scanner (and the official ParkSmarter app) resolve that code to
+the zone. No URL, no wrapper — the raw scanner code is enough.
+
+## Make one
+
+```bash
+qrencode -o my-zone.png -s 14 -m 4 "<ScannerCode>"
+```
+
+`<ScannerCode>` is the zone's `ScannerCode` field (visible on the meter detail
+screen, or in a `/api/Meter` response). For the DL zone in Sandpoint it's `DL`:
+
+```bash
+qrencode -o DL-sandpoint.png -s 14 -m 4 "DL"
+```
+
+![DL zone, Sandpoint](DL-sandpoint.png)
+
+## Which field to encode
+
+Use **`ScannerCode`** — it's what physical kiosk QR stickers encode and the only
+field the official ParkSmarter app accepts when scanning.
+
+- `ScannerCode` — ✅ works in BigBrainParking **and** the official app.
+- `ZoneName` (e.g. `dl`) — works as a fallback in BigBrainParking (case-sensitive
+  server-side; the app tries the lowercased form too).
+- `TerminalSerNo` (e.g. `30004246`) — uniquely identifies the terminal and works
+  in BigBrainParking, but the **official app rejects a bare serial** ("not
+  recognized as a ParkSmarter zone"), so don't use it for cross-app QR codes.
+
+When BigBrainParking scans a code it tries, in order: `ScannerCode → ZoneName →
+zonename(lowercased) → TerminalSerNo`, each independently, so any of these
+resolve.