Add header to PostThread view; update navigation to include stacking so that each tab maintains its own browsing history

src/view/lib/assets.native.ts

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+import {ImageSourcePropType} from 'react-native'
+
+export const AVIS: Record<string, ImageSourcePropType> = {
+  'alice.com': require('../../../public/img/alice.jpg'),
+  'bob.com': require('../../../public/img/bob.jpg'),
+  'carla.com': require('../../../public/img/carla.jpg'),
+}

src/view/lib/assets.ts

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+import {ImageSourcePropType} from 'react-native'
+
+export const AVIS: Record<string, ImageSourcePropType> = {
+  'alice.com': {uri: '/img/alice.jpg'},
+  'bob.com': {uri: '/img/bob.jpg'},
+  'carla.com': {uri: '/img/carla.jpg'},
+}