feat(water-log): surface irrigator name on log form + success screen

Makes the currently-signed-in irrigator visible while data is being
entered and confirmed, so the operator can verify the reading is
attributed to them at a glance.

  - LogForm nav bar: title + initials avatar + name as a subtitle,
    sign-out button moves from the form footer into the nav bar
    (more discoverable, frees vertical space for the Submit button)
  - SuccessScreen recap: 'Logged by {name}' row with avatar below
    the timestamp, matches the log form styling
  - MobileWaterApp threads irrigatorName through to both screens

The headgate list still shows 'Signed in as {name}' as a small
caption — left untouched to avoid duplicate UI.

Validation:
  - tsc --noEmit clean
  - eslint clean on all modified files

src/components/water/mobile/MobileWaterApp.tsx

@@ -213,6 +213,7 @@ export function MobileWaterApp() {
         <MobileWaterLogForm
           key={`${selectedHeadgate.id}-${logFormKey}`}
           headgate={selectedHeadgate}
+          irrigatorName={irrigatorName ?? undefined}
           onSubmit={handleLogSubmit}
           onChangeHeadgate={handleBackToHeadgates}
           onLogout={handleLogout}
@@ -230,6 +231,7 @@ export function MobileWaterApp() {
           measurement={submittedLog.measurement}
           unit={submittedLog.unit}
           timestamp={submittedLog.timestamp}
+          irrigatorName={irrigatorName ?? undefined}
           onLogAnother={handleLogAnother}
           onBackToHeadgates={handleBackToHeadgates}
         />