Add MTGA Assistant: native Linux deck and game tracker

PySide6 app that follows MTG Arena's Player.log under Steam/Proton and shows
my remaining library with draw odds, my zones, and everything revealed about
the opponent (board, graveyard, exile, known hand/library, seen this match).

- Auto-detects the Steam library, Proton prefix log and MTGA install
- Card names/costs/types from MTGA's local SQLite card database
- Tracks card identity across zone changes, shuffles and Bo3 games
- Scryfall card images with a lazy, size-capped LRU disk cache
- Live log following with restart detection, plus --replay mode
- Flags log gaps where MTGA omits oversized game-state messages

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/mtga_assistant/ui/preview.py

@@ -0,0 +1,80 @@
+"""Large image of the hovered card, with a text fallback while it downloads."""
+
+from __future__ import annotations
+
+from PySide6.QtCore import QSize, Qt
+from PySide6.QtGui import QPixmap, QResizeEvent
+from PySide6.QtWidgets import QLabel, QSizePolicy, QVBoxLayout, QWidget
+
+from ..carddb import CardDB
+from ..images import ImageCache
+
+
+class CardPreview(QWidget):
+    def __init__(self, cards: CardDB, images: ImageCache, parent=None):
+        super().__init__(parent)
+        self.cards = cards
+        self.images = images
+        self.grp_id: int | None = None
+        self._pixmap: QPixmap | None = None
+
+        self.image = QLabel()
+        self.image.setAlignment(Qt.AlignmentFlag.AlignCenter)
+        self.image.setSizePolicy(QSizePolicy.Policy.Ignored, QSizePolicy.Policy.Ignored)
+        self.image.setMinimumSize(200, 280)
+        self.caption = QLabel()
+        self.caption.setWordWrap(True)
+        self.caption.setAlignment(Qt.AlignmentFlag.AlignCenter)
+
+        lay = QVBoxLayout(self)
+        lay.setContentsMargins(4, 4, 4, 4)
+        lay.addWidget(self.image, 1)
+        lay.addWidget(self.caption)
+
+        images.image_ready.connect(self._on_image_ready)
+        self._show_placeholder("Hover a card to preview it")
+
+    def show_card(self, grp_id: int) -> None:
+        if grp_id == self.grp_id:
+            return
+        self.grp_id = grp_id
+        info = self.cards.get(grp_id)
+        details = [info.type_line]
+        if info.power or info.toughness:
+            details.append(f"{info.power}/{info.toughness}")
+        if info.set_code:
+            details.append(f"{info.set_code} #{info.collector_number}")
+        self.caption.setText(f"<b>{info.name}</b><br>{' · '.join(d for d in details if d)}")
+        pm = self.images.pixmap(grp_id)
+        if pm is not None:
+            self._pixmap = pm
+            self._rescale()
+        elif self.images.is_failed(grp_id):
+            self._show_placeholder("No image available")
+        else:
+            self._show_placeholder("Loading image…")
+
+    def _show_placeholder(self, text: str) -> None:
+        self._pixmap = None
+        self.image.setPixmap(QPixmap())
+        self.image.setText(text)
+
+    def _on_image_ready(self, grp_id: int) -> None:
+        if grp_id == self.grp_id:
+            self.grp_id = None
+            self.show_card(grp_id)
+
+    def _rescale(self) -> None:
+        if self._pixmap is None:
+            return
+        self.image.setPixmap(
+            self._pixmap.scaled(self.image.size(), Qt.AspectRatioMode.KeepAspectRatio,
+                                Qt.TransformationMode.SmoothTransformation)
+        )
+
+    def resizeEvent(self, event: QResizeEvent) -> None:
+        super().resizeEvent(event)
+        self._rescale()
+
+    def sizeHint(self) -> QSize:
+        return QSize(320, 600)