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/llm/src/model_manager.rs

@@ -220,7 +220,7 @@ pub fn recommend_tier(total_ram_bytes: u64, total_vram_bytes: Option<u64>) -> Ll
 }
 
 pub fn model_dir() -> PathBuf {
-    kon_core::paths::app_paths().llm_models_dir()
+    magnotia_core::paths::app_paths().llm_models_dir()
 }
 
 pub fn model_path(id: LlmModelId) -> PathBuf {
@@ -301,7 +301,7 @@ where
         .unwrap_or(0);
 
     let client = reqwest::Client::builder()
-        .user_agent("kon/0.1.0")
+        .user_agent("magnotia/0.1.0")
         .connect_timeout(std::time::Duration::from_secs(30))
         .build()
         .map_err(|e| DownloadError::Http(e.to_string()))?;