chore: rebrand from Kon/Corbie to Magnotia

Replace all instances of the legacy product names "Kon" and "Corbie" with
"Magnotia" across user-facing copy, code identifiers, package names, bundle
ids, file paths, and documentation. Preserves the unrelated "konsole" (KDE
terminal) reference and the parent CORBEL company name.

- Renames 10 Rust crates (kon-* → magnotia-*) and the tauri binary
- Updates package.json, tauri.conf.json (productName + identifier)
- Renames CSS classes (kon-rh-* → magnotia-rh-*) and animations
- Renames brand and roadmap docs
- Regenerates Cargo.lock and package-lock.json

Verified: svelte-check passes; pure-rust crates compile under new names.

crates/core/src/error.rs

@@ -4,12 +4,12 @@ use serde::Serialize;
 
 use crate::types::ModelId;
 
-/// Structured error type for Kon.
+/// Structured error type for Magnotia.
 ///
 /// Implements `Serialize` so errors can be sent to the frontend as
 /// structured JSON rather than opaque strings.
 #[derive(Debug, thiserror::Error, Serialize)]
-pub enum KonError {
+pub enum MagnotiaError {
     #[error("model not found: {0}")]
     ModelNotFound(ModelId),
 
@@ -57,4 +57,4 @@ fn serialize_io_error<S: serde::Serializer>(
     s.serialize_str(&err.to_string())
 }
 
-pub type Result<T> = std::result::Result<T, KonError>;
+pub type Result<T> = std::result::Result<T, MagnotiaError>;