refactor: rename internal Fai* design system + fai_ helpers to chain

The Studio design system, widgets and helpers carried a Fai* / fai_
prefix (FaiSpace, FaiColors, FaiTheme, FaiLog, 17 fai_*.dart files, the
faiBinary* l10n keys). Studio is the Ch∆In product, so rename them to
Chain* / chain_ — carefully preserving English fail/failure/failed.
Also fix stale references: the 'fai' binary in l10n strings -> 'chain',
FAI_* env vars (FAI_BIN/DATA_DIR/MODULES_DIR/TODAY/BOOTSTRAP_TOKEN) ->
CHAIN_*, fai_platform -> fai_chain, fai_hub -> chain_hub. Vendor
security-hook tooling (FAI_BANNED_TERMS_FILE) + the .fai bundle ext left.
flutter analyze + test: clean (20 passed).

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

docs/landing-page-design.md

@@ -102,7 +102,7 @@ Instead a `_DocReaderSheet` (modal bottom-sheet, same shape as
 existing `flutter_markdown` package — same one the Store uses
 for module READMEs. The doc source is bundled with Studio as
 `assets/docs/*.md`, kept in sync via a build step that copies
-the relevant files from `fai_platform/docs/`.
+the relevant files from `fai_chain/docs/`.
 
 Pros:
 - Air-gap-tauglich (no network call to read docs)
@@ -158,7 +158,7 @@ phased PRs:
    content with hard-coded strings (with i18n). Lands
    without the embedded-doc reader.
 2. **Phase B** — bundled markdown docs + `_DocReaderSheet`.
-   Pick 4-5 source docs from `fai_platform/docs/architecture/`
+   Pick 4-5 source docs from `fai_chain/docs/architecture/`
    that map to operator concerns (security, host-services,
    flow-experience, compliance-gaps). Add a build script that
    syncs them into `assets/docs/`.