feat(SPA): i18n internationzalization, english and french

features/spa_internationalization.feature

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+Feature: English and French throughout the SPA
+
+  Background:
+    Given all existing SPA interface text supports English and French
+    And English preserves existing copy and French provides equivalent meaning
+    And user content, API data and transactional emails remain unchanged
+    And visual checks are waived by the user for this slice
+    And the language control uses existing design tokens
+
+  Scenario Outline: Resolve the language before rendering
+    Given the browser-local stored language is <stored>
+    When the SPA starts
+    Then the interface language and document lang are <locale>
+    And browser language does not override the choice
+
+    Examples:
+      | stored      | locale |
+      | absent      | en     |
+      | en          | en     |
+      | fr          | fr     |
+      | unsupported | en     |
+      | unreadable  | en     |
+
+  Scenario Outline: Open the language picker in the profile submenu
+    Given the user opens the <menu>
+    When they activate the localized Language control
+    Then a compact scrollable picker offers English and Français
+    And the selected language is visibly and accessibly identified
+    And the picker uses Nuxt UI and existing design tokens
+
+    Examples:
+      | menu                    |
+      | desktop UserMenu        |
+      | mobile AccountMenuSheet |
+
+  Scenario: Switch language without losing work
+    Given the user is editing an unsaved display name on /profile
+    When they select Français
+    Then all SPA interface text immediately uses French
+    And the document lang is fr
+    And the route, session, theme and unsaved work remain unchanged
+    And no reload, navigation or API request is caused by switching
+    When they select English
+    Then the interface immediately restores English with the same preserved state
+
+  Scenario: Remember language on this browser
+    Given the user selected Français
+    When they navigate, reload, log out or return on this browser
+    Then French remains selected including on public authentication screens
+    And no language preference is sent to the server
+
+  Scenario: Storage failure does not prevent switching
+    Given preference storage cannot be written
+    When the user selects Français
+    Then French is used for the current session without an error
+    And a fresh load without a readable preference defaults to English
+
+  Scenario: Translate the entire interface
+    Given French is selected
+    When an existing SPA screen or shared component renders
+    Then navigation, forms, validation, notifications, dialogs and accessibility labels use French
+    And counts, human-readable dates and numbers use the selected locale
+    And Nuxt UI controls use the selected locale
+    And English and French catalogs have matching keys and interpolation parameters
+    And an unavailable French message falls back to English
+    And user content, account details, filenames, URLs, API data and emails remain unchanged
+
+  Scenario Outline: Operate the picker accessibly
+    Given the picker is open in the <menu>
+    When the user navigates its options with the keyboard and confirms a choice
+    Then the language changes and the picker closes
+    And focus returns to its language control
+    When they reopen it and press Escape or click outside
+    Then the picker closes without changing language
+
+    Examples:
+      | menu                    |
+      | desktop UserMenu        |
+      | mobile AccountMenuSheet |