feat(mail): default email_seed → \$PROJROOT/bait/ when unset

When service_cfg["email_seed"] is absent, compose_fragment now falls
back to $PROJROOT/bait/ if that directory exists on the host. Lets
operators drop a deployment-wide bait corpus into one place without
threading email_seed through every decky's config. Missing dir keeps
old no-op behavior.

decnet/services/pop3.py

@@ -3,10 +3,23 @@ from decnet.services.base import BaseService
 
 TEMPLATES_DIR = Path(__file__).parent.parent / "templates" / "pop3"
 
+# See decnet/services/imap.py for the same default-seed-dir rationale.
+_PROJ_ROOT = Path(__file__).resolve().parents[2]
+_DEFAULT_SEED_DIR = _PROJ_ROOT / "bait"
 
 _SEED_CONTAINER_PATH = "/var/spool/decnet-emails/seed"
 
 
+def _resolve_seed_path(service_cfg: dict | None) -> str | None:
+    if service_cfg:
+        seed = service_cfg.get("email_seed")
+        if seed:
+            return str(Path(str(seed)).expanduser().resolve())
+    if _DEFAULT_SEED_DIR.is_dir():
+        return str(_DEFAULT_SEED_DIR.resolve())
+    return None
+
+
 class POP3Service(BaseService):
     name = "pop3"
     ports = [110, 995]
@@ -15,6 +28,7 @@ class POP3Service(BaseService):
     #   email_seed: host path to a directory of .eml/.json files OR a
     #               single .json/.eml.  Mounted read-only; entries
     #               concatenate with the hardcoded bait list.
+    # Default fallback: $PROJROOT/bait/ when present.
 
     def compose_fragment(self, decky_name: str, log_target: str | None = None, service_cfg: dict | None = None) -> dict:
         fragment: dict = {
@@ -25,14 +39,12 @@ class POP3Service(BaseService):
         }
         if log_target:
             fragment["environment"]["LOG_TARGET"] = log_target
-        if service_cfg:
-            seed = service_cfg.get("email_seed")
-            if seed:
-                host_path = str(Path(str(seed)).expanduser().resolve())
-                fragment["environment"]["POP3_EMAIL_SEED"] = _SEED_CONTAINER_PATH
-                fragment.setdefault("volumes", []).append(
-                    f"{host_path}:{_SEED_CONTAINER_PATH}:ro"
-                )
+        host_path = _resolve_seed_path(service_cfg)
+        if host_path:
+            fragment["environment"]["POP3_EMAIL_SEED"] = _SEED_CONTAINER_PATH
+            fragment.setdefault("volumes", []).append(
+                f"{host_path}:{_SEED_CONTAINER_PATH}:ro"
+            )
         return fragment
 
     def dockerfile_context(self) -> Path | None: