perf(desktop): switch to the mimalloc global allocator

glibc keeps freed pages in its arena, so RSS did not fall back after
the webview was destroyed. mimalloc returns freed memory to the OS,
letting the resident footprint settle right after the UI closes.

Cargo.lock

@@ -1177,6 +1177,7 @@ dependencies = [
  "axum",
  "cursor-server",
  "libc",
+ "mimalloc",
  "rfd",
  "serde",
  "serde_json",
@@ -3417,6 +3418,15 @@ version = "0.2.16"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b6d2cec3eae94f9f509c767b45932f1ada8350c4bdb85af2fcab4a3c14807981"
 
+[[package]]
+name = "libmimalloc-sys"
+version = "0.1.49"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6a45a52f43e1c16f667ccfe4dd8c85b7f7c204fd5e3bf46c5b0db9a5c3c0b8e9"
+dependencies = [
+ "cc",
+]
+
 [[package]]
 name = "libredox"
 version = "0.1.19"
@@ -3604,6 +3614,15 @@ dependencies = [
  "autocfg",
 ]
 
+[[package]]
+name = "mimalloc"
+version = "0.1.52"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2d4139bb28d14ad1facf21d5eb8825051b326e172d216b39f6d31df53cc97862"
+dependencies = [
+ "libmimalloc-sys",
+]
+
 [[package]]
 name = "mime"
 version = "0.3.17"