chore(deps): bump dirs 5 -> 6 across all crates

Zero API changes, only bumps internal deps (windows-sys, redox_users).
Eliminates duplicate dirs versions in Cargo.lock.

Closes #257

cc

crates/qbz-cache/Cargo.toml

@@ -13,5 +13,5 @@ log = "0.4"
 serde = { version = "1", features = ["derive"] }
 
 # File system utilities
-dirs = "5"
+dirs = "6"
 filetime = "0.2"