refactor: centralize constants, eliminate unwraps, remove dead code, resolve TODOs

- R13+R16: Replace .expect() with .context()? in main.rs and identity.rs
- R17+R18+R19: Fix unwrap() calls in helpers and js-engine
- R20+R21: Remove #[allow(dead_code)] annotations and delete truly dead code
- R22-R26: Create constants.rs module, replace 21 hardcoded values across 12 files
- R28+R29: LND/DWN timeouts already present — verified
- R30-R33: Remove TODO comments, implement marketplace payment check

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/js-engine/src/lib.rs

@@ -127,7 +127,7 @@ impl ModuleLoader for ModsLoader {
         if referrer.contains("embassy") {
             bail!("Embassy.js cannot import anything else");
         }
-        let s = resolve_import(specifier, referrer).unwrap();
+        let s = resolve_import(specifier, referrer)?;
         Ok(s)
     }
 
@@ -246,7 +246,10 @@ impl JsExecutionEnvironment {
             }
         };
         let safer_handle = spawn_local(|| self.execute(procedure_name, input, variable_args)).await;
-        let output = safer_handle.await.unwrap()?;
+        let output = safer_handle.await.map_err(|e| {
+            tracing::error!("JS execution task panicked: {}", e);
+            (JsError::Engine, format!("JS execution task failed: {}", e))
+        })??;
         match serde_json::from_value(output.clone()) {
             Ok(x) => Ok(x),
             Err(err) => {