Fix greeter session start and login flow

greetd only launches the session after the greeter process exits. Quit
on successful StartSession instead of sitting on "Starting session…"
until SIGTERM.

Empty Secret/Visible answers are Some("") (not PAM cancel). The greetd
actor stays up across connect failure and reconnects. Escape cancels
the conversation. Session env sets XDG_SESSION_TYPE/DESKTOP. .desktop
parsing honors Hidden/NoDisplay/TryExec and quoted Exec=. Invalid TOML
warns instead of failing silent. Font, date, and Ken Burns config match
what the greeter actually draws.

breadgreet.example.toml

@@ -9,7 +9,8 @@
 mode = "color"
 path = ""
 blur = false
-# (Ken Burns is a breadlock-only background effect; not used by the greeter.)
+# Slow Ken Burns pan on image backgrounds (gentle drift + zoom). Opt-in: the
+# background redraws continuously at a low frame rate.
 ken_burns = false
 
 [clock]