feat(create): history viewer — revision timeline display (#40)

Add SiloHistoryViewer widget that opens in an MDI subwindow when the
user double-clicks the History node in the Silo tree. Displays revision
cards newest-first with revision number, Catppuccin-themed lifecycle
status badges, author, timestamp, and commit comment.

Changes:
- silo_viewers.py: SiloHistoryViewer with revision card layout,
  status badge QSS, scroll area, empty-history placeholder

Closes #40

src/Mod/Create/silo_viewers.py

@@ -423,6 +423,131 @@ class SiloMetadataEditor(QtWidgets.QWidget):
         event.accept()
 
 
+# ---------------------------------------------------------------------------
+#   History Viewer
+# ---------------------------------------------------------------------------
+
+_BADGE_STYLES = {
+    "draft": "background: #45475a; color: #cdd6f4;",
+    "review": "background: #f9e2af; color: #11111b;",
+    "released": "background: #a6e3a1; color: #11111b;",
+    "obsolete": "background: #f38ba8; color: #11111b;",
+}
+
+
+class SiloHistoryViewer(QtWidgets.QWidget):
+    """Read-only revision timeline from ``silo/history.json``."""
+
+    WINDOW_TITLE = "Silo \u2014 History"
+
+    def __init__(self, obj, parent=None):
+        super().__init__(parent)
+        self.setObjectName(f"SiloViewer_{obj.Name}")
+        self._build_ui(obj.RawContent)
+
+    def _build_ui(self, raw_content):
+        try:
+            data = json.loads(raw_content)
+        except Exception:
+            data = {}
+        revisions = data.get("revisions", [])
+
+        outer = QtWidgets.QVBoxLayout(self)
+        outer.setContentsMargins(16, 16, 16, 16)
+        outer.setSpacing(12)
+
+        # Header
+        header = QtWidgets.QHBoxLayout()
+        title = QtWidgets.QLabel("Revision History")
+        font = title.font()
+        font.setPointSize(font.pointSize() + 2)
+        font.setBold(True)
+        title.setFont(font)
+        header.addWidget(title)
+        header.addStretch()
+        outer.addLayout(header)
+
+        # Separator
+        line = QtWidgets.QFrame()
+        line.setFrameShape(QtWidgets.QFrame.HLine)
+        line.setFrameShadow(QtWidgets.QFrame.Sunken)
+        outer.addWidget(line)
+
+        if not revisions:
+            placeholder = QtWidgets.QLabel("No revision history available.")
+            placeholder.setAlignment(QtCore.Qt.AlignCenter)
+            outer.addWidget(placeholder)
+            outer.addStretch()
+            return
+
+        # Scroll area with revision cards
+        scroll = QtWidgets.QScrollArea()
+        scroll.setWidgetResizable(True)
+        scroll.setFrameShape(QtWidgets.QFrame.NoFrame)
+        content = QtWidgets.QWidget()
+        cards_layout = QtWidgets.QVBoxLayout(content)
+        cards_layout.setContentsMargins(0, 0, 0, 0)
+        cards_layout.setSpacing(0)
+
+        for i, rev in enumerate(revisions):
+            cards_layout.addWidget(self._make_revision_card(rev))
+            if i < len(revisions) - 1:
+                sep = QtWidgets.QFrame()
+                sep.setFrameShape(QtWidgets.QFrame.HLine)
+                sep.setFrameShadow(QtWidgets.QFrame.Sunken)
+                cards_layout.addWidget(sep)
+
+        cards_layout.addStretch()
+        scroll.setWidget(content)
+        outer.addWidget(scroll, 1)
+
+    def _make_revision_card(self, rev):
+        """Build a widget for a single revision entry."""
+        card = QtWidgets.QWidget()
+        layout = QtWidgets.QVBoxLayout(card)
+        layout.setContentsMargins(4, 8, 4, 8)
+        layout.setSpacing(4)
+
+        # Top line: Rev N · status badge · author · timestamp
+        top = QtWidgets.QHBoxLayout()
+        top.setSpacing(8)
+
+        rev_num = rev.get("revision", "?")
+        rev_label = QtWidgets.QLabel(f"<b>Rev {rev_num}</b>")
+        top.addWidget(rev_label)
+
+        status = rev.get("status", "draft")
+        badge = QtWidgets.QLabel(status)
+        style = _BADGE_STYLES.get(status, _BADGE_STYLES["draft"])
+        badge.setStyleSheet(
+            f"QLabel {{ {style} border-radius: 4px; "
+            f"padding: 1px 6px; font-size: 11px; }}"
+        )
+        top.addWidget(badge)
+
+        author = rev.get("author", "")
+        if author:
+            top.addWidget(QtWidgets.QLabel(f"\u00b7 {author}"))
+
+        timestamp = rev.get("timestamp", "")
+        if timestamp:
+            display_ts = _format_value("created_at", timestamp)
+            top.addWidget(QtWidgets.QLabel(f"\u00b7 {display_ts}"))
+
+        top.addStretch()
+        layout.addLayout(top)
+
+        # Comment line
+        comment = rev.get("comment", "")
+        if comment:
+            comment_label = QtWidgets.QLabel(comment)
+            comment_label.setWordWrap(True)
+            comment_label.setTextInteractionFlags(QtCore.Qt.TextSelectableByMouse)
+            layout.addWidget(comment_label)
+
+        return card
+
+
 # ---------------------------------------------------------------------------
 #   Viewer factory
 # ---------------------------------------------------------------------------
@@ -430,6 +555,7 @@ class SiloMetadataEditor(QtWidgets.QWidget):
 _VIEWER_REGISTRY = {
     "silo/manifest.json": SiloManifestViewer,
     "silo/metadata.json": SiloMetadataEditor,
+    "silo/history.json": SiloHistoryViewer,
 }