fix: render adaptive-sized preview by screenshotting the .card element

Previously full_page=True captured document.body.scrollHeight which on
short posts left ~2000px of empty viewport padding at the bottom
(screenshot was 1640×2400 for a 400px-tall card). For long posts it
also included the body's max-height padding.

Switch to element-level screenshot via page.locator('.card').screenshot()
when full_page is enabled. The card's bounding box drives the output
size, so short posts produce compact previews and long posts still
capture the entire card with no empty space.

Element screenshot failure (rare) falls back to page.screenshot() with
the original full_page logic.

main.py

@@ -948,12 +948,29 @@ class LinuxDoPreviewPlugin(Star):
 
             page.wait_for_timeout(300)
 
+            # ── 自适应截图：优先对 .card 元素截图，避免全页导致的巨大空白 ──
+            # 元素截图按内容的实际边界拍，零空白；否则回退到全页模式。
+            card_locator = page.locator(".card")
             full_page = self.config.get("screenshot_full_page", True)
-            page.screenshot(
-                path=str(save_path),
-                full_page=full_page,
-                timeout=timeout_ms,
-            )
+            try:
+                if full_page and card_locator.count() > 0:
+                    card_locator.first.screenshot(
+                        path=str(save_path),
+                        timeout=timeout_ms,
+                    )
+                else:
+                    page.screenshot(
+                        path=str(save_path),
+                        full_page=full_page,
+                        timeout=timeout_ms,
+                    )
+            except Exception:
+                # 回退：若元素截图失败（少见），退到全页截图
+                page.screenshot(
+                    path=str(save_path),
+                    full_page=full_page,
+                    timeout=timeout_ms,
+                )
             sz = save_path.stat().st_size
             logger.info(
                 f"[LinuxDoPreview] 渲染截图: {save_path.name} ({sz / 1024:.1f} KB)"