Widget system Phase 2: full cutover to widget-based rendering

device.py's mode-keyed dispatch is replaced by a real compositor:
load a frame's widgets, compute pixel rects via app/grid.py, render
each through its widget module, and composite with render_panel.
Physical NEXT/BACK buttons now execute each frame's assigned
FrameButtonAction rows instead of one hardcoded per-mode action.

api_frames.py, manage.py, and common.py's build_manage_content are
repointed to read/write the frame's widget config rows instead of
the old Frame columns, and every settings page (Photos/Calendar/
Whiteboard tabs) now pre-fills its form from the same widget config
the write endpoints actually save to -- previously the read and
write sides would have silently diverged. The old mode selector and
photo-inlay checkbox are removed along with their now-inert wiring;
arbitrary widget placement subsumes what the fixed inlay split did.

Ships together with Phase 1 (per-type render/action modules) since
splitting the read/write cutover across deploys would have left
settings changes with no visible effect.

server/app/templates/frame_config.html

@@ -2,7 +2,6 @@
 
 {% block title %}{{ frame.name or "Frame" }} · Configuration{% endblock %}
 {% block page_title %}{% include "_frame_name_edit.html" %}{% endblock %}
-{% block mode_picker %}{% include "_frame_mode_picker.html" %}{% endblock %}
 
 {% block device_status %}{% include "_device_status_bar.html" %}{% endblock %}
 {% block tabs %}{% include "_frame_tabs.html" %}{% endblock %}