firmware: zero-copy JPEG → BGR888 → DSI hot path; Scrypted pre-rotates

Architectural rework of the /frame hot path. Scrypted now ships every
JPEG already scaled + rotated to the panel's native dimensions (read
from the firmware's /state response so nothing is hardcoded on the
Scrypted side); the firmware decodes the JPEG straight into a BGR888
buffer that's directly draw_bitmap'able by the DSI driver, with zero
CPU pixel work and zero rotation work in between.

Firmware
- jpeg_decoder now uses JPEG_DECODE_OUT_FORMAT_RGB888 +
  JPEG_DEC_RGB_ELEMENT_ORDER_BGR. Output buffer sized for 800*480*3.
- New display_present_bgr888() is a one-liner that hands the decoder's
  output straight to esp_lcd_panel_draw_bitmap.
- /frame handler validates dimensions against the panel-native
  VIEWPORT_PANEL_WIDTH x VIEWPORT_PANEL_HEIGHT (was effective_dims
  branching on orientation). Returns 400 if it's anything else.
- /state JSON adds panel_width + panel_height so Scrypted can read them
  without hardcoding board-specific knowledge.
- display_present_rgb565 + s_rot_buf stay for the local-screens cold
  path (info screen, loading) which still does its own CPU conversion +
  rotation — infrequent enough that it's not worth the rewrite.

Scrypted
- startStream() GETs /state at stream-start time, caches panel_width
  and panel_height in viewport storage, and uses them as the ffmpeg
  scale target. Falls back to cached or 800x480 if /state is mid-reboot.
- For portrait viewports the ffmpeg pipeline now does
  scale=H:W:flags=lanczos,transpose=1 so the JPEG arrives pre-rotated
  90° CW into panel-native dimensions. Landscape is just scale=W:H.
- No more in-firmware rotation; Scrypted is the single source of truth
  for "how do I get this camera frame into a panel-shaped JPEG".

Expected ceiling lift: ~5 fps → ~10 fps, gated by the JPEG decoder
hardware throughput instead of the CPU rgb565→bgr888 + rotation loop.

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