build: drop the secp256k1-frost-kmp submodule

The previous commit moved the ChillDKG ritual onto bitcoin-kmp's native
implementation, which was the only thing in this app using this library. Nothing
imports `ac.cord.auxiliary.frost` or `ac.cord.auxiliary.cryptography` any more, so
the submodule, its composite build and the dependency on it all go.

Removed:
  - the `secp256k1-frost-kmp` submodule (deinit, git rm, and .git/modules cleared)
  - its entry in .gitmodules
  - `includeBuild("secp256k1-frost-kmp")` in settings.gradle.kts, with the
    surrounding comment made singular now that one composite build remains
  - `implementation("ac.cord.auxiliary:library:1.0.0")` in composeApp

Note that `ac/cord/auxiliary/compose/**` under composeApp/src/jvmMain is this
app's own code in a similarly-named package, unrelated to the library and
untouched.

Two commits made on the submodule to keep it building inside this composite --
gating its ios targets on macos, and moving it to AGP 9.4.0 -- existed only in
this repository's .git/modules copy and are destroyed by the removal. They were
exported as patches first. Neither is a loss worth chasing: both existed purely to
make that project cooperate with a composite build that no longer includes it.

The library is not gone from the world, only from this build. It still holds a
FROST signer, which is the obvious next need now that the ritual produces
threshold key material. But bitcoin-kmp ships `fr.acinq.bitcoin.crypto.frost.Frost`
against the same natives this app now packages, so the signer will almost
certainly come from there rather than from a second copy of secp256k1.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

composeApp/build.gradle.kts

@@ -126,9 +126,6 @@ kotlin {
 
             implementation("com.ionspin.kotlin:bignum:0.3.10")
 
-            // resolved via the secp256k1-frost-kmp composite build (see settings.gradle.kts)
-            implementation("ac.cord.auxiliary:library:1.0.0")
-
             implementation("no.synth:kmp-zip:0.8.0")
             implementation("no.synth:kmp-zip-okio:0.8.0")
         }