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/styles.css

@@ -232,10 +232,7 @@ select option {
   width: 34px;
   height: 34px;
   border-radius: 10px;
-  background: linear-gradient(150deg, var(--accent-hover), var(--accent-active));
-  color: #fff;
-  font-weight: 800;
-  font-size: 19px;
+  background: linear-gradient(150deg, var(--bg-elev), var(--bg-rail));
   display: flex;
   align-items: center;
   justify-content: center;
@@ -543,15 +540,13 @@ select option {
   gap: 14px;
 }
 .empty-mark {
-  width: 72px;
-  height: 72px;
-  border-radius: 20px;
+  width: 88px;
+  height: 88px;
+  border-radius: 22px;
   display: flex;
   align-items: center;
   justify-content: center;
-  font-size: 36px;
-  color: #fff;
-  background: linear-gradient(150deg, var(--accent-hover), var(--accent-active));
+  background: linear-gradient(150deg, var(--bg-elev), var(--bg-rail));
   box-shadow: 0 10px 30px var(--accent-glow);
 }
 .empty-state h2 {