Add a static image widget (PNG/JPEG/GIF/BMP/WEBP/TIFF/PDF upload)

A new widget type showing a single user-uploaded image with no live
upstream to poll -- decoded once at upload time (PDF's first page via
pypdfium2, BSD-3-Clause/Apache-2.0, no copyleft exposure) into plain RGB
PNG bytes, then composed per a crop/stretch/letterbox display mode like
the photos widget.

server/tests/test_widgets_static.py

@@ -0,0 +1,74 @@
+"""app.widgets.static_image -- unit-level, no HTTP: constructs Widget/
+StaticWidgetConfig rows directly. The decode-at-upload-time path (see
+app/image_upload.py) is covered separately in test_image_upload.py and
+the HTTP-level upload endpoint in
+test_widget_config_and_queue_endpoints.py -- these tests only exercise
+render(), which always starts from already-decoded PNG bytes."""
+
+from __future__ import annotations
+
+import io
+import time
+
+from PIL import Image
+
+from app import widgets
+from app.models import Frame, StaticWidgetConfig, Widget
+
+
+def _png_bytes(size=(40, 30), color=(200, 50, 50)) -> bytes:
+    buf = io.BytesIO()
+    Image.new("RGB", size, color).save(buf, format="PNG")
+    return buf.getvalue()
+
+
+def _make_widget(db_session, **cfg_kwargs) -> tuple[Frame, Widget]:
+    frame = db_session.get(Frame, 1)
+    widget = Widget(frame_id=frame.id, widget_type="static", x=0, y=0, w=2, h=2,
+                    sort_order=0, created_at=time.time())
+    db_session.add(widget)
+    db_session.flush()
+    db_session.add(StaticWidgetConfig(widget_id=widget.id, **cfg_kwargs))
+    db_session.commit()
+    return frame, widget
+
+
+def test_render_shows_a_placeholder_when_no_image_uploaded(db_session):
+    frame, widget = _make_widget(db_session)
+    img = widgets.static_image.render(db_session, frame, widget, 300, 200)
+    assert img.size == (300, 200)
+    assert img.mode == "RGB"
+
+
+def test_render_composes_the_uploaded_image(db_session):
+    frame, widget = _make_widget(db_session, image=_png_bytes())
+    img = widgets.static_image.render(db_session, frame, widget, 300, 200)
+    assert img.size == (300, 200)
+    assert img.mode == "RGB"
+
+
+def test_render_respects_display_mode_stretch_fill(db_session):
+    frame, widget = _make_widget(db_session, image=_png_bytes(), display_mode="stretch_fill")
+    img = widgets.static_image.render(db_session, frame, widget, 300, 200)
+    assert img.size == (300, 200)
+
+
+def test_render_respects_display_mode_letterbox(db_session):
+    frame, widget = _make_widget(db_session, image=_png_bytes(size=(100, 10)), display_mode="letterbox")
+    img = widgets.static_image.render(db_session, frame, widget, 300, 200)
+    assert img.size == (300, 200)
+
+
+def test_render_at_minimum_grid_footprint(db_session):
+    """grid.MIN_FOOTPRINT["static"] is (1, 1) cells -- on an 8x5 grid
+    against a full 800x480 panel that's a 100x96 box, the smallest a
+    static widget can actually be placed at."""
+    frame, widget = _make_widget(db_session, image=_png_bytes())
+    img = widgets.static_image.render(db_session, frame, widget, 100, 96)
+    assert img.size == (100, 96)
+
+
+def test_no_button_actions():
+    """A fixed uploaded image -- nothing to advance/back/check."""
+    assert widgets.static_image.ACTIONS == {}
+    assert widgets.static_image.ACTION_LABELS == {}