Widget system Phase 3: calendar widgets become size-aware

calendar_render.py's _build_* functions now take a real target box
and pick font sizes/margins from three discrete size tiers (nearest
pixel-area fit) instead of always laying out at full panel size and
resizing after the fact -- a calendar widget placed smaller than the
full panel gets an actually-legible layout instead of shrunk text.
Month view falls back to agenda below the smallest tier, where 7
columns can no longer stay readable.

The old photo-inlay split (inlay_region/_content_region/_paste_inlay)
is deleted along with it -- arbitrary widget placement already
subsumes what a fixed half-panel split did, and every call site has
passed photo_inlay=None since the Phase 2 cutover.

Also adds HTTP-level test coverage for GET .../preview/calendar,
which had none before this -- it's what caught a stale photo_inlay
kwarg left over from the _build signature change that would have
TypeError'd on every request.

server/app/face_labels.py

@@ -39,13 +39,13 @@ def compute_face_labels(preview_bytes: bytes, faces: list[dict], display_mode: s
     computed here won't match what's actually on screen.
 
     `region` is (x0, y0, w, h): where in the logical canvas the photo
-    actually landed, if not the whole thing -- e.g. calendar mode's
-    agenda photo-inlay only occupies half the panel (see
-    calendar_render.inlay_region), and without this a label would be
-    placed as if the photo filled the entire canvas, landing well off
-    where the inlaid photo actually is. None (the default) means the
-    photo fills the whole logical canvas, matching every other caller
-    (photos mode always renders full-panel).
+    actually landed, if not the whole thing -- e.g. a photo widget placed
+    in one corner of the panel rather than full-screen (see
+    routers/common.py's build_manage_content, which passes each photo
+    widget's own placement rect) -- without this a label would be placed
+    as if the photo filled the entire canvas, landing well off where the
+    widget actually is. None (the default) means the photo fills the
+    whole logical canvas.
 
     The placement math matches render_frame()'s own composition step
     exactly (see image_pipeline._placement_transform, shared so the two