Themes as standalone config files + configurable shadow + dim unfocused + resize content-loss fix

- Theme is now a runtime struct loaded from its own TOML file
  (theme = "themes/dark.toml" in config.toml, like panels), all
  fields optional with built-in defaults; ships themes/dark.toml and
  themes/light.toml
- shadow = true/false in config.toml, shadow_color per theme
- dim_unfocused (per theme): unfocused window content is dimmed
- BUGFIX: resizing a window no longer erases its content and history —
  the vt100 parser was recreated on every resize; now uses
  screen_mut().set_size() which preserves the grid and scrollback
- New integration check: window content survives mouse resize (20/20)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/integration.py

@@ -213,6 +213,27 @@ def main():
               f"frame-text: {text[-200:]!r}")
         app.send({"type": "close_window", "window_id": emoji_open["id"]})
 
+        # ── 4b2. Resize får inte radera fönstrets innehåll ───────────────
+        app.send({"type": "spawn_window",
+                  "command": "printf 'RESIZE-KEEP\\n'; exec cat",
+                  "request_id": "w-resize"})
+        rz_open = app.recv_until("window_opened")
+        disp.frame_text(timeout=5.0, contains="RESIZE-KEEP")
+        app.send({"type": "list_windows", "request_id": "lr"})
+        wlr = app.recv_until("window_list")
+        rw = next(w for w in wlr["windows"] if w["id"] == rz_open["id"])
+        # dra i nedre högra hörnet: krymp fönstret 4 kolumner/2 rader
+        corner_x = rw["x"] + rw["width"] - 1
+        corner_y = rw["y"] + rw["height"] - 1
+        disp.send(mouse_event({"Down": "Left"}, corner_x, corner_y))
+        disp.send(mouse_event({"Drag": "Left"}, corner_x - 4, corner_y - 2))
+        disp.send(mouse_event({"Up": "Left"}, corner_x - 4, corner_y - 2))
+        time.sleep(0.5)
+        text = disp.frame_text(timeout=5.0, contains="RESIZE-KEEP")
+        check("innehållet överlever fönster-resize", "RESIZE-KEEP" in text,
+              f"frame-tail: {text[-200:]!r}")
+        app.send({"type": "close_window", "window_id": rz_open["id"]})
+
         # ── 4c. Scrollback: mushjul utan mus-tracking scrollar historiken ─
         app.send({"type": "spawn_window",
                   "command": "seq 1 200; exec cat",