Pin flash size + a larger app partition in sdkconfig.defaults

The board's flash is 8MB, but neither flash size nor partition table were
pinned in sdkconfig.defaults, so a fresh clone would silently fall back to
IDF's default (2MB flash, ~1MB "single app" partition table) rather than
what this project was actually being built/tested against. Building against
that default left only 4% of the app partition free before the HTTP client
work (task 6) even lands. Adds a custom partitions.csv with a 2MB app
partition and pins CONFIG_ESPTOOLPY_FLASHSIZE_8MB so the committed config
reproduces a working build for anyone else who clones this.

firmware/partitions.csv

@@ -0,0 +1,4 @@
+# Name,   Type, SubType, Offset,  Size,     Flags
+nvs,      data, nvs,     0x9000,  0x6000,
+phy_init, data, phy,     0xf000,  0x1000,
+factory,  app,  factory, 0x10000, 0x200000,

firmware/sdkconfig.defaults

@@ -1 +1,9 @@
 CONFIG_LWIP_MAX_SOCKETS=16
+
+# ESP32-C6 dev board has 8MB flash. A 2MB app partition (vs. the ~1MB
+# "single app" default) gives headroom for the HTTP client, TLS, and image
+# buffers still to come, without needing to fight for space on every build.
+CONFIG_ESPTOOLPY_FLASHSIZE_8MB=y
+CONFIG_PARTITION_TABLE_CUSTOM=y
+CONFIG_PARTITION_TABLE_CUSTOM_FILENAME="partitions.csv"
+CONFIG_PARTITION_TABLE_FILENAME="partitions.csv"