docs(auth): what building the apps' side settled (ADR 0017)

ADR 0017's step-3 section: codes by mail sign nobody in, the ways in
for accounts without a working password, changes while signed in,
sessions ended in every service, the apps' screens (the QR package and
the alternatives considered), the keys in the keyring, anonymous
people's keys waiting for step 4, and libsodium on devices (results
copied before they leave an isolate; a C compiler and make for every
build). The module's and halkyon_app's READMEs and the AGENTS files say
the same; the snap's build installs make for sodium's build hook.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

snap/snapcraft.yaml

@@ -117,11 +117,13 @@ parts:
     # runtime brings the same GStreamer (1.24) to run. The video player
     # (media_kit, for Jellyfin) links libmpv, which neither has: the Snap
     # brings Ubuntu's libmpv2 with what it needs (ffmpeg's libraries), or the
-    # app wouldn't start.
+    # app wouldn't start. sodium's build hook compiles libsodium with its
+    # configure script and make (ADR 0017), with the clang above.
     build-packages:
       - libgstreamer1.0-dev
       - libgstreamer-plugins-base1.0-dev
       - libmpv-dev
+      - make
     stage-packages:
       - libmpv2
     override-build: |