feat: i18n switcher in app-frontend (#4990)

* feat: app i18n stuff

* feat: locale switching on load

* feat: db migration

* feat: polish + fade indicator impl onto TabbedModal

* fix: prepr checks

* fix: remove staging lock for language switching

* fix: lint

.github/instructions/i18n-convert.instructions.md

@@ -2,7 +2,7 @@
 applyTo: '**/*.vue'
 ---
 
-You are given a Nuxt/Vue single-file component (.vue). Your task is to convert every hard-coded natural-language string in the <template> into our localization system using @vintl/vintl-nuxt (which wraps FormatJS).
+You are given a Nuxt/Vue single-file component (.vue). Your task is to convert every hard-coded natural-language string in the <template> into our localization system using vue-i18n with utilities from `@modrinth/ui`.
 
 Please follow these rules precisely:
 
@@ -13,40 +13,53 @@ Please follow these rules precisely:
 
 2. Create message definitions
 
-- In the <script setup> block, import `defineMessage` or `defineMessages` from `@vintl/vintl`.
+- In the <script setup> block, import `defineMessage` or `defineMessages` from `@modrinth/ui`.
 - For each extracted string, define a message with a unique `id` (use a descriptive prefix based on the component path, e.g. `auth.welcome.long-title`) and a `defaultMessage` equal to the original English string.
   Example:
   const messages = defineMessages({
   welcomeTitle: { id: 'auth.welcome.title', defaultMessage: 'Welcome' },
-  welcomeDescription: { id: 'auth.welcome.description', defaultMessage: 'You’re now part of the community…' },
+  welcomeDescription: { id: 'auth.welcome.description', defaultMessage: 'You're now part of the community…' },
   })
 
 3. Handle variables and ICU formats
 
 - Replace dynamic parts with ICU placeholders: "Hello, ${user.name}!" → `{name}` and defaultMessage: 'Hello, {name}!'
-- For numbers/dates/times, use ICU/FormatJS options (e.g., currency): `{price, number, ::currency/USD}`
+- For numbers/dates/times, use ICU options (e.g., currency): `{price, number, ::currency/USD}`
 - For plurals/selects, use ICU: `'{count, plural, one {# message} other {# messages}}'`
 
 4. Rich-text messages (links/markup)
 
 - In `defaultMessage`, wrap link/markup ranges with tags, e.g.:
   "By creating an account, you agree to our <terms-link>Terms</terms-link> and <privacy-link>Privacy Policy</privacy-link>."
-- Render rich-text messages with `<IntlFormatted>` from `@vintl/vintl/components` and map tags via `values`:
-  <IntlFormatted
-    :message="messages.tosLabel"
-    :values="{
-      'terms-link': (chunks) => <NuxtLink to='/terms'>{chunks}</NuxtLink>,
-      'privacy-link': (chunks) => <NuxtLink to='/privacy'>{chunks}</NuxtLink>,
-    }"
-  />
-- For simple emphasis: `'Welcome to <strong>Modrinth</strong>!'` and map `'strong': (c) => <strong>{c}</strong>`
+- Render rich-text messages with `<IntlFormatted>` from `@modrinth/ui` using named slots:
+  <IntlFormatted :message-id="messages.tosLabel">
+    <template #terms-link="{ children }">
+      <NuxtLink to="/terms">
+        <component :is="() => children" />
+      </NuxtLink>
+    </template>
+    <template #privacy-link="{ children }">
+      <NuxtLink to="/privacy">
+        <component :is="() => children" />
+      </NuxtLink>
+    </template>
+  </IntlFormatted>
+- For simple emphasis: `'Welcome to <strong>Modrinth</strong>!'` with a slot:
+  <template #strong="{ children }">
+    <strong><component :is="() => children" /></strong>
+  </template>
+- For more complex child handling, use `normalizeChildren` from `@modrinth/ui`:
+  <template #bold="{ children }">
+    <strong><component :is="() => normalizeChildren(children)" /></strong>
+  </template>
 
 5. Formatting in templates
 
-- Import and use `useVIntl()`; prefer `formatMessage` for simple strings:
+- Import and use `useVIntl()` from `@modrinth/ui`; prefer `formatMessage` for simple strings:
   `const { formatMessage } = useVIntl()`
   `<button>{{ formatMessage(messages.welcomeTitle) }}</button>`
-- Vue methods like `$formatMessage`, `$formatNumber`, `$formatDate` are also available if needed.
+- Pass variables as a second argument:
+  `{{ formatMessage(messages.greeting, { name: user.name }) }}`
 
 6. Naming conventions and id stability
 
@@ -58,7 +71,8 @@ Please follow these rules precisely:
 
 8. Update imports and remove literals
 
-- Ensure imports for `defineMessage`/`defineMessages`, `useVIntl`, and `<IntlFormatted>` are present. Replace all hard-coded strings with `formatMessage(...)` or `<IntlFormatted>` and remove the literals.
+- Ensure imports from `@modrinth/ui` are present: `defineMessage`/`defineMessages`, `useVIntl`, `IntlFormatted`, and optionally `normalizeChildren`.
+- Replace all hard-coded strings with `formatMessage(...)` or `<IntlFormatted>` and remove the literals.
 
 9. Preserve functionality