[ethanndickson]

This started as bumping the protoc plugins in the flake. They come from a third-party grpc-swift flake that hasn't been touched since December 2024, and since #276 it needs its own ancient nixpkgs pin just to evaluate on Darwin. nixpkgs doesn't package protoc-gen-grpc-swift, and grpc-swift stopped attaching prebuilt plugins to v1 releases, so keeping v1 meant maintaining our own Nix package for it.

Instead, this PR requires macOS 15 so we can move to grpc-swift v2. Its build tool plugin generates the Swift code for the .proto files at build time, building protoc and both plugins from the resolved packages. The generated code can't drift from the runtime anymore, and the committed .pb.swift/.grpc.swift files, make proto, and all of the protoc tooling in the flake are gone. Clean builds do compile protoc from source once.

The protos now live in a local Swift package, Coder-Desktop/Proto, with a VPNProto target for vpn.proto and a MutagenProto target for the Mutagen protos and their gRPC clients. Attached to an Xcode target, the plugin only sees files in a build phase, which meant putting every .proto and .json in the sources phase and adding no-op build rules to stop Xcode warning about them. In a package it just picks up the files in the target directory. VPNLib imports MutagenProto with internal import, so the Mutagen types stay out of its public API, and anything using the Vpn_ types now imports VPNProto. The exact versions are still pinned in project.yml, and Package.swift only sets minimums. Each target also has a Swift file containing only a comment, since SwiftPM won't load a target without one and the generated code doesn't count.

v2's bidirectional streams are scoped to a closure, so the prompting stream is now withPrompter { promptID in ... } rather than a stream we cancel in a defer. v2 also fails RPCs straight away whilst the channel is backing off after a failed connect, which broke the startup probe (the socket doesn't exist yet), so that call now sets waitForReady. FileSyncDaemonTests caught this, as they run against the real Mutagen binary.

The appcast's minimumSystemVersion and the cask's depends_on are bumped to 15 too, so macOS 14 users don't get offered an update that won't launch.

I also bumped the rest of the app dependencies whilst I was here: Sparkle 2.10.1 (its updater APIs are now @MainActor, so UpdaterService is too), Subprocess 4.0.1, URLRouting 0.7.0, SDWebImageSwiftUI 3.1.4, SDWebImageSVGCoder 1.8.0, and the SwiftLintPlugins floor to 0.65.1 to match the flake's swiftlint. URLRouting 0.7.0 deprecates the .case(CoderRoute.open(...)) form we use; the replacement needs @CasePathable, which needs a direct swift-case-paths dependency, so I've left that warning for now.

This is on top of the stack (#276, #277, #275). I've tested it locally with Xcode 27.

[matifali]

I wonder if we can migarte to mise from nix here?