build(i18n): embed the craft-i18n catalogs pinned in craft-i18n.rev

Adds the craft-i18n crate (craft-libs f7cc8a7, storytold/craft-libs#247)
and the build.rs recipe from craft-i18n's docs/integration.md: with
CRAFT_I18N_DIR set, CRAFT_I18N embeds each locale's common.tsv and
wordcraft.tsv (paths via {:?}, never canonicalized); unset, it is empty
and the build is English only; CRAFT_I18N_REQUIRED=1 makes that an
error. craft-i18n.rev pins storytold/craft-i18n be1f053 (LF via
.gitattributes); the flatpak manifest checks out the same commit and
sets CRAFT_I18N_DIR.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

packaging/linux/flatpak/ai.storyteller.wordcraft.yml

@@ -43,6 +43,9 @@ modules:
       env:
         CARGO_HOME: /run/build/wordcraft/cargo
         CARGO_NET_OFFLINE: 'true'
+        # The interface translations (the craft-i18n source below), embedded by build.rs.
+        CRAFT_I18N_DIR: /run/build/wordcraft/craft-i18n
+        CRAFT_I18N_REQUIRED: '1'
     build-commands:
       - cargo --offline build --release --locked -p wordcraft -p wordcraft-cli
       - install -Dm755 target/release/wordcraft /app/bin/wordcraft
@@ -64,4 +67,9 @@ modules:
           - corpus
           - build-flatpak
           - .flatpak-builder
+      # The interface translations at the commit in craft-i18n.rev (a test keeps the two equal).
+      - type: git
+        url: https://github.com/storytold/craft-i18n
+        commit: be1f0535afbae9bfa1554b9afd545690eb2de2a5
+        dest: craft-i18n
       - cargo-sources.json