feat: freeze frame at playhead

Enable the timeline toolbar freeze-frame button: capture the composited
canvas as PNG, add it as an image media asset, split the video element
at the playhead, shift the right part by the freeze duration, and insert
a 3-second image element in the gap. Adds RendererManager.captureFrame(),
reused by createSnapshot().

Also adds the Nix devshell (flake.nix/flake.lock: bun + nodejs) and
AGENTS.md repository guidelines.

1 files changed, 32 insertions, 0 deletions

diff --git a/flake.nix b/flake.nix
new file mode 100644
index 00000000..2cf1c096
--- /dev/null
+++ b/flake.nix
@@ -0,0 +1,32 @@
+{
+  description = "OpenCut Classic development shell";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  outputs =
+    { nixpkgs, ... }:
+    let
+      systems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "aarch64-darwin"
+      ];
+      forEachSystem = nixpkgs.lib.genAttrs systems;
+    in
+    {
+      devShells = forEachSystem (
+        system:
+        let
+          pkgs = import nixpkgs { inherit system; };
+        in
+        {
+          default = pkgs.mkShell {
+            packages = with pkgs; [
+              bun
+              nodejs
+            ];
+          };
+        }
+      );
+    };
+}