feat: decrypt copied ciphertext inside the keyboard

[bglglzd]

Summary

• add an embedded Encrypt / Decrypt switch behind the IME shield
• decrypt one explicitly copied, bounded CB1 ciphertext inside the protected keyboard panel
• render plaintext in a drawing-only, non-selectable secure view and never commit it to the host app or clipboard
• support secure reply to the same contact, Vault unlock, lifecycle cleanup, and compact landscape layouts
• prepare version 0.3.0 and update the security/release documentation

Verification

• full JVM unit suite: PASS
• Android connected tests on API 36: 7/7 app and 1/1 native PASS
• Android lint debug/release: PASS
• Rust fmt, clippy -D warnings, tests, cargo audit: PASS
• parser fuzz crate fmt/clippy/test/audit: PASS
• Kotlin style, security source scan, release script regression tests: PASS
• manual English/Russian IME layout QA in landscape at font scale 2.0: PASS

Honest gaps

• no paired-contact end-to-end exchange was performed on the emulator
• no physical GrapheneOS device run was performed
• the product has not received an independent security audit