Show a spinner while a screen starts and while a camera loads, bring a camera's first picture sooner, and round the alert's camera picture (0.2.87)

Starting screen: until the first layout, render() keeps the status text
("Connecting to Home Assistant", "Waiting for ESP Screens") and a spinner
as one block in the middle of the page; the first layout deletes both.
The room label stays empty meanwhile, and ROOM_NAME defaults to "".

Camera: the full screen shows a spinner instead of "Loading image". The
screen asks when the camera opens and loads a link as soon as it comes
(camera_load, never under a finger) instead of waiting for the 250 ms
tick twice, and online_image reads 16 KB per loop instead of 4 KB. On the
bench Guition with the EZVIZ camera: cold 5.8 -> 4.55 s, warm 3.4 -> 2.06 s.

Alert picture: the image takes the frame's radius (18, the alert card's);
LVGL's software renderer clips it row by row, without a layer.

One spinner_create() makes the busy sheet's, the starting screen's and the
camera's spinner; theme role CAMERA_TRACK colours the camera's ring. The
README gets a row of camera pictures; docs/CAMERA.md, RELEASING.md and
TEST_RESULTS_0287.md describe the measurements.

Checked with tools/check.sh (473 Python tests, 19/19 C++, editor) and
--firmware: CYD 1,678,816 B (91.5 %, +1,312 B), Guition 2,055,280 B
(25.3 %). Host renders of both boards in both looks; four OTA builds on
the bench Guition Wallbox, which runs this firmware.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/core.yaml

@@ -14,10 +14,10 @@
 # fonts/materialdesignicons-webfont.ttf (Apache 2.0, github.com/Templarian/MaterialDesign-Webfont).
 ############################################################
 substitutions:
-  SCREEN_FIRMWARE_VERSION: "0.2.72"
+  SCREEN_FIRMWARE_VERSION: "0.2.73"
   DASHBOARD_INBOX_INTERNAL: "false"
   CALIBRATION_ON_BOOT: "false"                     # a bench build: start on the touch test page
-  ROOM_NAME: "Choose tiles in HA"                  # shown top left until ESP Screens sends the screen name
+  ROOM_NAME: ""                                    # top left until ESP Screens sends the screen name
 
   ## DIM settings
   AUTO_DIM: "true"                                 # true = enable auto dimming