Store user state in binary CBOR format

The state structure is growing a little too complex to store it as a TOML.
User state is now stored at `~/.config/ncspot/userstate.cbor`.

Cargo.lock

@@ -1293,6 +1293,12 @@ dependencies = [
  "tracing-futures",
 ]
 
+[[package]]
+name = "half"
+version = "1.7.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "62aca2aba2d62b4a7f5b33f3712cb1b0692779a56fb510499d5c0aa594daeaf3"
+
 [[package]]
 name = "hashbrown"
 version = "0.9.1"
@@ -2029,6 +2035,7 @@ dependencies = [
  "reqwest 0.9.24",
  "rspotify",
  "serde",
+ "serde_cbor",
  "serde_json",
  "strum 0.20.0",
  "strum_macros 0.20.1",
@@ -3200,6 +3207,16 @@ dependencies = [
  "serde_derive",
 ]
 
+[[package]]
+name = "serde_cbor"
+version = "0.11.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e18acfa2f90e8b735b2836ab8d538de304cbb6729a7360729ea5a895d15a622"
+dependencies = [
+ "half",
+ "serde",
+]
+
 [[package]]
 name = "serde_derive"
 version = "1.0.125"