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>

tools/today/propose.sh

@@ -5,17 +5,17 @@
 # Defaults to a local Ollama at 127.0.0.1:11434 with gemma3:4b.
 # See README.md for env-var overrides.
 #
-# Output:  ~/.fai/today/proposals/<ISO-DATE>/candidate-<n>.yaml
+# Output:  ~/.chain/today/proposals/<ISO-DATE>/candidate-<n>.yaml
 # Operator then runs accept.sh with the chosen file.
 set -euo pipefail
 
 HERE="$(cd "$(dirname "$0")" && pwd)"
 DATE="$(date -u +%Y-%m-%d)"
-OUT_DIR="${FAI_TODAY_OUT:-$HOME/.fai/today/proposals/$DATE}"
-N="${FAI_TODAY_N:-3}"
-API="${FAI_TODAY_API:-http://127.0.0.1:11434/api/generate}"
-MODEL="${FAI_TODAY_MODEL:-gemma3:4b}"
-KEY="${FAI_TODAY_KEY:-}"
+OUT_DIR="${CHAIN_TODAY_OUT:-$HOME/.chain/today/proposals/$DATE}"
+N="${CHAIN_TODAY_N:-3}"
+API="${CHAIN_TODAY_API:-http://127.0.0.1:11434/api/generate}"
+MODEL="${CHAIN_TODAY_MODEL:-gemma3:4b}"
+KEY="${CHAIN_TODAY_KEY:-}"
 
 mkdir -p "$OUT_DIR"