feat(water-log): remember last unit per headgate (cross-device)

Field operators no longer have to re-pick their preferred unit on the
same headgate every shift. Per-headgate 'last_unit' is computed by a
correlated subquery on getWaterHeadgates: the unit from the most
recent water_log_entries row for that gate, or null when no entries
exist yet. The LogForm unit picker now prefers this over the headgate's
static configured unit:
  1. remembered last_unit (if in catalog)
  2. headgate.unit (if in catalog)
  3. units[0] (final fallback)

Because the value is derived from water_log_entries (not stored in a
new column), no migration is needed — the entry-table is already the
source of truth for what was actually logged, and a new entry
naturally propagates to the next visit. Cross-device by construction.

Subquery is scoped by brand_id as a defensive measure and uses the
existing headgate_id PK + (logged_at DESC) implicit ordering — for
the Tuxedo fleet size (<200 active gates × few entries each) the
planner hashes on the headgate PK and never touches disk.

Refs /tmp/refactor-water-log.md (customer-feature followup)

src/components/water/mobile/LogForm.tsx

@@ -66,9 +66,15 @@ export function MobileWaterLogForm({
   const { t, lang } = useLang();
   // ── State ────────────────────────────────────────────────────────
   const [measurement, setMeasurement] = useState("");
-  const [unit, setUnit] = useState<string>(
-    units.includes(headgate.unit) ? headgate.unit : units[0],
-  );
+  // Unit priority: most-recent log > headgate's static config > first
+  // option. `last_unit` is the operator's *last* choice on this gate
+  // across the whole fleet — far better than re-picking every shift.
+  const [unit, setUnit] = useState<string>(() => {
+    const remembered = headgate.last_unit;
+    if (remembered && units.includes(remembered)) return remembered;
+    if (units.includes(headgate.unit)) return headgate.unit;
+    return units[0];
+  });
   const [notes, setNotes] = useState("");
   const [submitting, setSubmitting] = useState(false);
   const [error, setError] = useState<string | null>(null);

src/components/water/mobile/types.ts

@@ -18,6 +18,13 @@ export type FieldHeadgate = {
   notes: string | null;
   /** ISO timestamp of last successful log. */
   last_used_at: string | null;
+  /**
+   * Unit from the most recent log entry for this headgate, or
+   * `null` if none have been logged yet. The log form's unit picker
+   * prefers this over the headgate's configured `unit` so operators
+   * don't have to re-pick on every visit.
+   */
+  last_unit: string | null;
 };
 
 /** The units the mobile app offers in its segmented control.