refactor(core,storage): deduplicate code patterns (TODO-014)

Extract PttCapability event construction helper in events.rs.
Extract ensure_dir() helper in chanora_storage. Add TODO(refactor)
annotations for patterns requiring shared crate or API changes.

crates/chanora_bridge/src/lib.rs

@@ -56,6 +56,10 @@ pub enum BridgeError {
     /// The caller submitted a malformed command DTO.
     #[error("invalid command: {0}")]
     InvalidCommand(String),
+    // TODO(refactor): DnsFailed and ServerRejected mirror ProtocolError variants
+    // in chanora_protocol. These cannot be unified without changing the public FFI
+    // API (flutter_rust_bridge generates Dart types from these). Revisit only if
+    // the bridge error types are being reworked.
     /// Hostname resolution failed. Distinct from `Connection` so the
     /// UI can show a meaningful "Server not found" message.
     #[error("dns: could not resolve '{host}': {reason}")]