breadgreet: skip the username step, fix clobbered typography

Login UX
- Enumerate human accounts from /etc/passwd (UID_MIN..UID_MAX per
  /etc/login.defs, real login shell, minus nobody) and skip the
  username field: one account goes straight to the password prompt
  with the name shown, several get a picker. Zero found falls back to
  typing. New [user] config: `name` pins one account, `prompt = true`
  restores the type-it flow.
- The greeter opens the greetd conversation at startup in auto mode so
  it lands directly on the password prompt (AutoStart).
- reset_to_username -> reset_auth: auto mode re-opens the password
  prompt for the same account on error/Escape rather than showing a
  username field it never had.

Typography fix
- bind_window_auto re-broadcasts the shared component sheet (incl. its
  `* { font-size }` base rule) at USER-10, which outranks the
  APPLICATION-priority provider apply_app_css uses regardless of
  selector specificity -- so the hero clock and every other type
  override silently collapsed to base size. Ride breadgreet's sheet
  through bind_window_auto_with_app_css instead (USER-9), matching
  breadbar.

Polish
- Accent rule between the clock and card, drawn in via scaleX with a
  glow halo; card gradient + top highlight + deep shadow, bg-pop
  overshoot entrance, slow bg-breathe idle pulse; layered accent focus
  glow on the entry; gradient session/user pills. Entrance/idle motion
  is CSS @keyframes now (setup_entrance removed).
- Errors pinned to a legible red -- BOS's default @red slot is a warm
  ochre, unreadable as a warning.
- Drop the duplicate prompt text (placeholder + status line both said
  "Password:").

Preview harness
- scripts/preview.sh + scripts/mock-greetd.py run the real greeter in a
  nested Weston window against a stand-in greetd, so the whole flow
  (spinner, wrong-password shake, success) is drivable without a
  reboot. $BREADGREET_CONFIG overrides the config search path so a
  preview never touches /etc/greetd/breadgreet.toml.

breadgreet.example.toml

@@ -21,6 +21,17 @@ date_format = "%A · %b %d"
 [font]
 family = "Varela Round"
 
+[user]
+# By default breadgreet enumerates the system's human accounts (/etc/passwd,
+# UID_MIN..UID_MAX from /etc/login.defs) and skips the username field: one
+# account goes straight to the password prompt, several show a picker. These
+# keys override that.
+#
+# Pin one account and don't enumerate (empty = auto-detect):
+name = ""
+# Always ask for the username by hand (the pre-enumeration behaviour):
+prompt = false
+
 [sessions]
 # Directories scanned for .desktop session entries, in order.
 wayland_dirs = ["/usr/share/wayland-sessions"]