Add third button: "scan to manage" QR overlay on the current photo

Pressing the manage button (GPIO1) overlays a small QR code -- "SCAN TO
MANAGE" -- in the top-right corner of whatever photo is currently on
screen, linking to the server's config page, then reverts to the plain
photo after 30 seconds.

The overlay is spliced into the existing streaming fetch as chunks pass
through (frame_client.c's http_read_fn), rather than buffering the full
192,000-byte frame in RAM: only the small overlay rectangle itself
(~30KB) is ever held in memory, generated via new stride-parameterized
drawing helpers (epd_draw_*_ex in epd_draw.c) that let the existing
QR/text drawing code target an arbitrarily-sized buffer instead of a
full-frame one. epd7in3e.c is untouched -- it has no idea an overlay
exists.

firmware/main/Kconfig.projbuild

@@ -121,4 +121,19 @@ menu "ESPresso Frame Configuration"
             FRAME_RESET_BUTTON_GPIO above; defaults to a different pin
             than the reset button. Set to -1 to disable the feature.
 
+    config FRAME_MANAGE_BUTTON_GPIO
+        int "Manage button GPIO (-1 to disable)"
+        default 1
+        range -1 7
+        help
+            Button wired between this GPIO and GND (active-low, internal
+            pull-up enabled in firmware -- no external resistor needed).
+            Pressing it wakes the device (if asleep), displays the
+            current photo with a small "scan to manage" QR code overlaid
+            in the top-right corner (linking to the tools server's config
+            page) for 30 seconds, then reverts to the plain photo. Must
+            be GPIO 0-7 for the same deep-sleep-wakeup reason as
+            FRAME_RESET_BUTTON_GPIO above; defaults to a different pin
+            than the other two buttons. Set to -1 to disable the feature.
+
 endmenu