fix: persist base_charge_control_end_threshold for one-shot charging

base_charge_control_end_threshold was marked #[serde(skip)] so it only
lived in memory. triggering one-shot charge then restarting asusd would
permanently lose the original charge limit.

- remove #[serde(skip)] so the field gets persisted
- add serde default of 0 for backwards compat (skips restore for
  upgraded configs missing the field)
- add comments explaining Default vs serde default asymmetry

asusd/src/config.rs

@@ -8,6 +8,12 @@ use serde::{Deserialize, Serialize};
 
 const CONFIG_FILE: &str = "asusd.ron";
 
+/// Default value for base_charge_control_end_threshold when not present in config.
+/// Returns 0 so restore_charge_limit() skips restoration for upgraded configs.
+fn default_base_charge_limit() -> u8 {
+    0
+}
+
 #[derive(Default, Clone, Deserialize, Serialize, PartialEq)]
 pub struct Tuning {
     pub enabled: bool,
@@ -19,8 +25,8 @@ type Tunings = HashMap<PlatformProfile, Tuning>;
 pub struct Config {
     // The current charge limit applied
     pub charge_control_end_threshold: u8,
-    /// Save charge limit for restoring
-    #[serde(skip)]
+    /// Save charge limit for restoring after one-shot full charge
+    #[serde(default = "default_base_charge_limit")]
     pub base_charge_control_end_threshold: u8,
     pub disable_nvidia_powerd_on_battery: bool,
     /// An optional command/script to run when power is changed to AC
@@ -86,6 +92,9 @@ impl Default for Config {
     fn default() -> Self {
         Self {
             charge_control_end_threshold: 100,
+            // NOTE: This is intentionally 100 (not 0 like the serde default).
+            // New installs get 100 (no limit). Upgraded configs missing this
+            // field get 0 via serde, which skips restore_charge_limit().
             base_charge_control_end_threshold: 100,
             disable_nvidia_powerd_on_battery: true,
             ac_command: Default::default(),