feat: expose prompt caching TTL setting in UI (#273)

Adds a dropdown to select ENABLE_PROMPT_CACHING_1H or FORCE_PROMPT_CACHING_5M env vars when starting Claude Code sessions. Requires Claude Code v2.1.108+.

src-tauri/src/config.rs

@@ -75,6 +75,11 @@ pub struct ClaudeStartOptions {
     /// Valid values: "low", "medium", "high", "xhigh" (Opus 4.7 only), "max". None uses CLI default.
     #[serde(default)]
     pub effort_level: Option<String>,
+
+    /// Sets `ENABLE_PROMPT_CACHING_1H=1` or `FORCE_PROMPT_CACHING_5M=1` env var (v2.1.108+).
+    /// Values: "1h" → 1-hour TTL, "5m" → force 5-minute TTL. None uses CLI default.
+    #[serde(default)]
+    pub prompt_caching_ttl: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -246,6 +251,11 @@ pub struct HikariConfig {
     /// Valid values: "low", "medium", "high", "xhigh" (Opus 4.7 only), "max". None uses CLI default.
     #[serde(default)]
     pub effort_level: Option<String>,
+
+    /// Sets `ENABLE_PROMPT_CACHING_1H=1` or `FORCE_PROMPT_CACHING_5M=1` env var (v2.1.108+).
+    /// Values: "1h" → 1-hour TTL, "5m" → force 5-minute TTL. None uses CLI default.
+    #[serde(default)]
+    pub prompt_caching_ttl: Option<String>,
 }
 
 impl Default for HikariConfig {
@@ -302,6 +312,7 @@ impl Default for HikariConfig {
             show_clear_context_on_plan_accept: true,
             custom_model_option: None,
             effort_level: None,
+            prompt_caching_ttl: None,
         }
     }
 }
@@ -520,6 +531,7 @@ mod tests {
             show_clear_context_on_plan_accept: true,
             custom_model_option: None,
             effort_level: None,
+            prompt_caching_ttl: None,
         };
 
         let json = serde_json::to_string(&config).unwrap();

src-tauri/src/wsl_bridge.rs

@@ -410,6 +410,15 @@ impl WslBridge {
                 }
             }
 
+            // Set prompt caching TTL env var if specified (v2.1.108+)
+            if let Some(ref ttl) = options.prompt_caching_ttl {
+                match ttl.as_str() {
+                    "1h" => { cmd.env("ENABLE_PROMPT_CACHING_1H", "1"); }
+                    "5m" => { cmd.env("FORCE_PROMPT_CACHING_5M", "1"); }
+                    _ => {}
+                }
+            }
+
             cmd
         } else {
             // Running on Windows - use wsl with bash login shell to ensure PATH is loaded
@@ -485,6 +494,15 @@ impl WslBridge {
                 }
             }
 
+            // Set prompt caching TTL env var if specified (v2.1.108+)
+            if let Some(ref ttl) = options.prompt_caching_ttl {
+                match ttl.as_str() {
+                    "1h" => { claude_cmd.push_str("ENABLE_PROMPT_CACHING_1H=1 "); }
+                    "5m" => { claude_cmd.push_str("FORCE_PROMPT_CACHING_5M=1 "); }
+                    _ => {}
+                }
+            }
+
             claude_cmd.push_str(
                 "claude --output-format stream-json --input-format stream-json --verbose",
             );