Move SearchState from app.py to search_state.py (no behavior change)

Step 8 of the gui/app.py + gui/preview.py structural refactor — first
move out of app.py. Pure copy: the SearchState dataclass moves to its
own module. app.py grows its first re-export shim block at the bottom
so any internal `SearchState(...)` reference in BooruApp keeps working
through the module-namespace lookup. Shim removed in commit 14.

booru_viewer/gui/app.py

@@ -51,18 +51,6 @@ from .settings import SettingsDialog
 log = logging.getLogger("booru")
 
 
-@dataclass
-class SearchState:
-    """Mutable state that resets on every new search."""
-    shown_post_ids: set[int] = field(default_factory=set)
-    page_cache: dict[int, list] = field(default_factory=dict)
-    infinite_exhausted: bool = False
-    infinite_last_page: int = 0
-    infinite_api_exhausted: bool = False
-    nav_page_turn: str | None = None
-    append_queue: list = field(default_factory=list)
-
-
 class LogHandler(logging.Handler, QObject):
     """Logging handler that emits to a QTextEdit."""
 
@@ -3606,3 +3594,7 @@ def run() -> None:
     window = BooruApp()
     window.show()
     sys.exit(app.exec())
+
+
+# -- Refactor compatibility shims (deleted in commit 14) --
+from .search_state import SearchState  # re-export for refactor compat

booru_viewer/gui/search_state.py

@@ -0,0 +1,17 @@
+"""Mutable per-search state container."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+
+
+@dataclass
+class SearchState:
+    """Mutable state that resets on every new search."""
+    shown_post_ids: set[int] = field(default_factory=set)
+    page_cache: dict[int, list] = field(default_factory=dict)
+    infinite_exhausted: bool = False
+    infinite_last_page: int = 0
+    infinite_api_exhausted: bool = False
+    nav_page_turn: str | None = None
+    append_queue: list = field(default_factory=list)