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>

tests/test_theme.py

@@ -111,7 +111,7 @@ class Paints(unittest.TestCase):
         tiles = (COMPONENT / 'runtime_tiles.h').read_text()
         for needle in ('lv_obj_add_style(w.slider,theme::style(theme::Paint::knob),LV_PART_KNOB);',
                        'lv_obj_add_style(w.busy,theme::style(theme::Paint::veil),0);',
-                       'lv_obj_add_style(w.spinner,theme::style(theme::Paint::spinner),LV_PART_MAIN);',
+                       'lv_obj_add_style(spinner, theme::style(theme::Paint::spinner), LV_PART_MAIN);',
                        'lv_obj_add_style(part, theme::style(theme::Paint::slate), 0);',
                        'inline void restyle() {'):
             self.assertIn(needle, tiles)