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>

src-tauri/tauri.conf.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
-  "productName": "MTG Library",
+  "productName": "mtgc",
   "version": "0.1.0",
   "identifier": "com.garrettmills.mtgc",
   "build": {
@@ -13,7 +13,7 @@
     "windows": [
       {
         "label": "main",
-        "title": "MTG Library",
+        "title": "mtgc",
         "width": 1200,
         "height": 800,
         "resizable": true