fix(nix): enable sandboxed builds with bun2nix (#199)

* fix(nix): enable sandboxed builds with bun2nix

The Nix package was broken on Linux because `bun install` requires
network access, which is blocked by Nix sandboxing (enabled by default
on Linux).

This switches to bun2nix for dependency management:
- Add bun2nix flake input to pre-fetch all npm dependencies
- Generate bun.nix lockfile for reproducible dependency resolution
- Copy bun cache to writable location during build to avoid EACCES
  errors from bunx writing to the read-only Nix store
- Add nanoid as an explicit dependency (was imported directly but only
  available as a transitive dep, which breaks with isolated linker)
- Update CI workflow to perform a full sandboxed build
- Add bun2nix to devShell for easy lockfile regeneration

Closes #197

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(nix): create writable workdir for database access

The app uses process.cwd()/data for the database path, but when running
from the Nix store the cwd is read-only. Create a writable working
directory with symlinks to app files and a real data directory.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

flake.lock

@@ -1,8 +1,50 @@
 {
   "nodes": {
+    "bun2nix": {
+      "inputs": {
+        "flake-parts": "flake-parts",
+        "import-tree": "import-tree",
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "systems": "systems",
+        "treefmt-nix": "treefmt-nix"
+      },
+      "locked": {
+        "lastModified": 1770895533,
+        "narHash": "sha256-v3QaK9ugy9bN9RXDnjw0i2OifKmz2NnKM82agtqm/UY=",
+        "owner": "nix-community",
+        "repo": "bun2nix",
+        "rev": "c843f477b15f51151f8c6bcc886954699440a6e1",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "bun2nix",
+        "type": "github"
+      }
+    },
+    "flake-parts": {
+      "inputs": {
+        "nixpkgs-lib": "nixpkgs-lib"
+      },
+      "locked": {
+        "lastModified": 1769996383,
+        "narHash": "sha256-AnYjnFWgS49RlqX7LrC4uA+sCCDBj0Ry/WOJ5XWAsa0=",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "rev": "57928607ea566b5db3ad13af0e57e921e6b12381",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "type": "github"
+      }
+    },
     "flake-utils": {
       "inputs": {
-        "systems": "systems"
+        "systems": "systems_2"
       },
       "locked": {
         "lastModified": 1731533236,
@@ -18,6 +60,21 @@
         "type": "github"
       }
     },
+    "import-tree": {
+      "locked": {
+        "lastModified": 1763762820,
+        "narHash": "sha256-ZvYKbFib3AEwiNMLsejb/CWs/OL/srFQ8AogkebEPF0=",
+        "owner": "vic",
+        "repo": "import-tree",
+        "rev": "3c23749d8013ec6daa1d7255057590e9ca726646",
+        "type": "github"
+      },
+      "original": {
+        "owner": "vic",
+        "repo": "import-tree",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1761672384,
@@ -34,8 +91,24 @@
         "type": "github"
       }
     },
+    "nixpkgs-lib": {
+      "locked": {
+        "lastModified": 1769909678,
+        "narHash": "sha256-cBEymOf4/o3FD5AZnzC3J9hLbiZ+QDT/KDuyHXVJOpM=",
+        "owner": "nix-community",
+        "repo": "nixpkgs.lib",
+        "rev": "72716169fe93074c333e8d0173151350670b824c",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "nixpkgs.lib",
+        "type": "github"
+      }
+    },
     "root": {
       "inputs": {
+        "bun2nix": "bun2nix",
         "flake-utils": "flake-utils",
         "nixpkgs": "nixpkgs"
       }
@@ -54,6 +127,42 @@
         "repo": "default",
         "type": "github"
       }
+    },
+    "systems_2": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    },
+    "treefmt-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "bun2nix",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1770228511,
+        "narHash": "sha256-wQ6NJSuFqAEmIg2VMnLdCnUc0b7vslUohqqGGD+Fyxk=",
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "rev": "337a4fe074be1042a35086f15481d763b8ddc0e7",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "type": "github"
+      }
     }
   },
   "root": "root",