chore: migrate from open-x4-sdk to freeink-sdk (#2449)

This PR moves us from the xteink openx4 SDK to the freeink sdk from
https://freeink.org. Out of the box there are NO changes needed in the
firmware to support this swap, it all magically works as is. However as
we support more than just the x3/x4 devices, this sdk allows us to pass
env vars into the build commands to include support for other devices.
As support for new hardware such as touch screens and bluetooth are
added the xteink builds decide at compile time if the libraries are used
or not. For example right now the freeinkui and icons libraries are in
the platform.io file but as they are not used anywhere, they won't be
included in the final build. Once the touch branch and sd themes branch
are merged in this sdk is required for them to function correctly. All
the docs for freeink are available at freeink.org/docs. x4/x3 is a
single binary build unlike other devices that will build unique binaries
for each device. Eventually we will want to remove a lot of the manual
isx3 type stuff from our firmware and go through the boardsupport api
the sdk provides as it will generalize everything into one common system
that any device can support. The upcoming touch branch does a lot of
this for us but this initial PR is JUST to get the sdk swapped over
without any code changes to show seamless integration without any
regressions.

docs/contributing/architecture.md

@@ -8,7 +8,7 @@ At a high level, it is firmware that uses an activity-driven application archite
 
 ```mermaid
 graph TD
-    A[Hardware: ESP32-C3 + SD + E-ink + Buttons] --> B[open-x4-sdk]
+    A[Hardware: ESP32-C3 + SD + E-ink + Buttons] --> B[freeink-sdk]
     B --> C[lib/hal wrappers]
     C --> D[src/main.cpp runtime loop]
     D --> E[Activities layer]
@@ -195,10 +195,10 @@ When editing related source assets, regenerate via normal build steps/scripts.
 - `src/`: app orchestration, settings/state, and activity implementations
 - `src/network/`: web server and OTA/update networking
 - `src/components/`: theming and shared UI components
-- `lib/hal/`: hardware abstraction wrappers around open-x4-sdk
+- `lib/hal/`: hardware abstraction wrappers around freeink-sdk
 - `lib/Epub/`: EPUB parser, layout, CSS handling, and hyphenation
 - `lib/`: supporting libraries (fonts, text, filesystem helpers, etc.)
-- `open-x4-sdk/`: hardware SDK submodule (display, input, storage, battery)
+- `freeink-sdk/`: hardware SDK submodule (display, input, storage, battery). Docs: https://freeink.org/docs
 - `docs/`: user and technical documentation
 
 ## Embedded constraints that shape design