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.

docs/issues/keystore-thread-safety.md

@@ -17,7 +17,7 @@ Additional details:
 
 - Stored keys now live in-memory only for the life of the process.
 - `retrieve_api_key` checks the in-memory keystore first, then falls
-  back to read-only `KON_API_KEY_<PROVIDER>` environment variables so
+  back to read-only `MAGNOTIA_API_KEY_<PROVIDER>` environment variables so
   externally injected secrets still work.
 - Module docs now describe the real tradeoff clearly: safe from any
   thread, but non-persistent until a proper OS keychain backend lands.