ci: enforce ui/→data/ layer boundary with check-layers task (A5)

Add a new `check-layers` Taskfile target that greps for any
`package:sharedinbox/data/` imports inside `lib/ui/` and exits non-zero
if any are found. It runs as part of `check-fast` so violations are
caught on every commit and in CI before they reach main.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Taskfile.yml

@@ -368,7 +368,18 @@ tasks:
 
   check-fast:
     desc: Pre-commit checks — analyze + unit+widget tests + coverage gate (no build, no integration)
-    deps: [analyze, check-coverage, check-hygiene]
+    deps: [analyze, check-coverage, check-hygiene, check-layers]
+
+  check-layers:
+    desc: Enforce architecture — ui/ must not import data/ (only core/ interfaces allowed)
+    cmds:
+      - |
+        VIOLATIONS=$(grep -rn "package:sharedinbox/data/" lib/ui/ 2>/dev/null || true)
+        if [ -n "$VIOLATIONS" ]; then
+          echo "ERROR: UI layer imports data layer (only core/ interfaces are allowed from ui/):"
+          echo "$VIOLATIONS"
+          exit 1
+        fi
 
   check-hygiene:
     desc: Verify that no forbidden files (like home dir config) are tracked