Skip redundant panel refreshes and fetch the image before the config check

The panel driver now splits writing a frame into its SPI buffer
(epd_write_frame(), which also computes a CRC32 as it streams) from
actually triggering the physical refresh (epd_turn_on_display()).
frame_client.c compares the new CRC against the last one that was
actually refreshed (persisted in NVS) and skips the refresh entirely
when they match -- e.g. a reboot redisplaying the same photo before the
server's refresh interval elapsed no longer causes a visible flash for
no visual change.

Also reorders the per-wake fetch cycle: the image fetch (15s timeout)
now goes before the config check (3s timeout), instead of after. The
config check's tighter timeout was intermittently tripping on
connection-setup latency that's common on the first request after
waking from a long deep sleep (e.g. stale ARP); putting the more
tolerant request first absorbs that latency, and the config check then
rides the connection it already warmed up.

firmware/components/epd7in3e/include/epd7in3e.h

@@ -40,6 +40,29 @@ typedef size_t (*epd_read_fn_t)(uint8_t *chunk, size_t chunk_size, void *ctx);
  */
 esp_err_t epd_display_stream(epd_read_fn_t read_fn, void *ctx);
 
+/**
+ * Like epd_display_stream(), but writes the frame into the panel's
+ * internal buffer over SPI WITHOUT triggering the physical refresh (the
+ * visible flash/flicker, which also takes 15-30+ seconds) -- call
+ * epd_turn_on_display() separately to make it visible. Returns
+ * ESP_ERR_INVALID_SIZE if read_fn didn't supply exactly EPD_FRAME_BYTES,
+ * same as epd_display_stream(); either way nothing is refreshed, so the
+ * visible screen is left untouched on error.
+ *
+ * If out_crc32 is non-NULL, it's set to a CRC32 of the bytes written --
+ * lets a caller compare against the last-displayed frame's CRC and skip
+ * the refresh entirely when nothing actually changed (e.g. redisplaying
+ * the same photo after a reboot).
+ */
+esp_err_t epd_write_frame(epd_read_fn_t read_fn, void *ctx, uint32_t *out_crc32);
+
+/**
+ * Triggers the panel's physical refresh cycle (power on, refresh, power
+ * off) -- the visible flash/flicker sequence, 15-30+ seconds. Call after
+ * epd_write_frame() to make the written buffer visible.
+ */
+esp_err_t epd_turn_on_display(void);
+
 /** Convenience wrapper around epd_display_stream() for an in-memory frame buffer. */
 esp_err_t epd_display_buffer(const uint8_t *frame, size_t len);