merge: clear the clippy backlog and unify lock helpers (D1-warnings, D3)

51 clippy warnings -> 0, with 8 justified #[allow]s (IPC arity, specta wire
types, and the 9 test-only await-holding-lock sites). 27 raw lock calls moved to
the poison-tolerant helpers - all of them test code; production was already
clean.

Caught a non-neutral clippy --fix: removing the redundant 'use hostname;' in
credentials.rs orphaned its #[cfg(target_os = "linux")] onto SERVICE_NAME,
which would have cfg'd the constant out of every non-Linux build. Compiles clean
on Linux, so only Windows/macOS CI would have caught it.

src-tauri/src/credentials.rs

@@ -20,9 +20,6 @@ use sha2::{Digest, Sha256};
 use std::fs;
 use std::path::PathBuf;
 
-#[cfg(target_os = "linux")]
-use hostname;
-
 #[cfg(not(target_os = "android"))]
 const SERVICE_NAME: &str = "com.dtourolle.jellytau";
 
@@ -203,15 +200,12 @@ impl CredentialStore {
 
         // secret-tool doesn't support --version, so we test with a search command
         // that will succeed even if no items are found
-        match Command::new("secret-tool")
+        Command::new("secret-tool")
             .arg("search")
             .arg("service")
             .arg("__nonexistent_test__")
             .output()
-        {
-            Ok(_) => true,   // If command runs (even with no results), secret-tool is available
-            Err(_) => false, // Command not found or can't execute
-        }
+            .is_ok()
     }
 
     #[cfg(all(not(target_os = "android"), not(target_os = "linux")))]