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>

README.md

@@ -66,7 +66,7 @@ cargo tauri build
 
 Outputs (under the workspace `target/release/`):
 
-- **`target/release/mtgc-app`** — the standalone executable. The web UI and all
+- **`target/release/mtgc`** — the standalone executable. The web UI and all
   app logic are baked into this one file. It dynamically links the system WebKit/GTK
   libraries (`webkit2gtk4.1`, `gtk3`), so those must be present on the machine that
   runs it — they are on any modern desktop Linux and are installed by
@@ -77,12 +77,12 @@ Outputs (under the workspace `target/release/`):
 Useful variants:
 
 ```bash
-cargo tauri build --no-bundle          # only the bare target/release/mtgc-app binary
+cargo tauri build --no-bundle          # only the bare target/release/mtgc binary
 ```
 
 Notes:
 
-- The `mtgc-app` binary is a single file but relies on the system WebKit/GTK libs
+- The `mtgc` binary is a single file but relies on the system WebKit/GTK libs
   being present (installed by `devel-setup.sh`).
 - The binary does **not** embed card data — first run still downloads the Scryfall
   bulk into the OS app-data dir, and your synced `library.db` lives there by default
@@ -115,3 +115,14 @@ cargo build                  # build everything incl. the desktop app
 The core and frontend are platform-agnostic; `cargo tauri icon` already emitted
 iOS/Android icon sets. Porting is adding `tauri ios` / `tauri android` targets —
 only reference-data provisioning differs (download on-device).
+
+## Branding
+
+The logo is an original pirate-ship silhouette in the terracotta brand color
+(`#C8553D`) — a nod to the MTG card "Pirate Ship", not a reproduction of its
+art. Source: `src-tauri/icons/logo-source.svg`. To regenerate the full
+platform icon set after editing it:
+
+```bash
+cargo tauri icon src-tauri/icons/logo-source.svg
+```