build(android): bump uniffi 0.28 -> 0.31.2 to drop unmaintained bincode

cargo-deny flagged bincode 1.3.3 as unmaintained (RUSTSEC-2025-0141). bincode is
NOT a workspace dependency — no crate declares or uses it; it was pulled purely as
a build-time transitive dep of uniffi_macros 0.28. Rather than ignore the advisory,
upgrading uniffi to the current 0.31.2 removes bincode from the tree entirely
(along with other stale transitive deps: askama_escape, mime/mime_guess, unicase).

`cargo deny check advisories` now passes with no new ignore. The proc-macro
surface (setup_scaffolding!, #[uniffi::export], the uniffi-bindgen CLI) is
unchanged across 0.28->0.31, so the generated Kotlin NesController API is
identical. Verified: cargo test/clippy green (7 mobile tests); the debug APK
builds with the 0.31 bindgen and boots clean on the Z Fold 7.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: doublegate