Rebrand to mtgc: pirate-ship logo, lowercase app name, mtgc binary

- New logomark: an original pirate-ship silhouette (hull, twin billowing
  sails, swallow-tail pennant) in the terracotta brand gradient on a dark
  square — a nod to the MTG card "Pirate Ship", not a reproduction of its
  art. Source at src-tauri/icons/logo-source.svg; regenerate the full
  platform icon set (desktop/Windows Store/iOS/Android) with
  `cargo tauri icon src-tauri/icons/logo-source.svg`.
- Apply it in-app via a new inline ShipLogo component (App.tsx), replacing
  the old "M" sidebar mark and "✦" empty-state mark with dark rounded
  squares containing the ship glyph.
- Rename the app to lowercase "mtgc" everywhere: window title and
  productName (tauri.conf.json), <title> (index.html), sidebar brand text,
  empty-state heading.
- Rename the Tauri package/binary mtgc-app -> mtgc (and its lib crate
  mtgc_app_lib -> mtgc_lib), so `cargo tauri build` now produces
  target/release/mtgc; updated main.rs and every README reference.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

index.html

@@ -3,7 +3,7 @@
   <head>
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
-    <title>MTG Library Catalog</title>
+    <title>mtgc</title>
   </head>
   <body>
     <div id="root"></div>