feat: overhaul Nix flake with full Linux support and convenience commands

- Remove rust-overlay dependency, use nixpkgs built-in Rust
- Add comprehensive Linux library dependencies for Chromium browsers
- Set up proper LD_LIBRARY_PATH/NIX_LD for NixOS dynamic linking
- Add nix run apps: setup, dev, tauri-dev, full-dev, build, test, info
- Add AppImage release launcher for NixOS users (v0.17.6)
- Add flake-test CI workflow
- Update release version and hashes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

flake.lock

@@ -37,28 +37,7 @@
     "root": {
       "inputs": {
         "flake-utils": "flake-utils",
-        "nixpkgs": "nixpkgs",
-        "rust-overlay": "rust-overlay"
-      }
-    },
-    "rust-overlay": {
-      "inputs": {
-        "nixpkgs": [
-          "nixpkgs"
-        ]
-      },
-      "locked": {
-        "lastModified": 1767926800,
-        "narHash": "sha256-x0n73J6ufD/EhDlVdcoAmF0OQHZ+b0a2cKDc8RZyt+o=",
-        "owner": "oxalica",
-        "repo": "rust-overlay",
-        "rev": "499e9eed88ff9494b6604205b42847e847dfeb91",
-        "type": "github"
-      },
-      "original": {
-        "owner": "oxalica",
-        "repo": "rust-overlay",
-        "type": "github"
+        "nixpkgs": "nixpkgs"
       }
     },
     "systems": {