Windows port Phase 0: platform-select the audio backend

Make the tree compile for Windows without touching core logic, by
confining all Linux/PipeWire assumptions behind cfg gates and a single
platform-selected backend alias. No new dependencies — the cpal/WASAPI
backend lands in Phase 1; this ships a no-op stub.

- Cargo.toml: move pipewire + rfd(xdg-portal) under cfg(unix); add a
  cfg(windows) rfd using the Win32 dialog backend.
- audio: gate pipewire_impl to unix, add a cpal_impl stub for windows,
  and select between them via the new PlatformAudioBackend alias.
- core: use PlatformAudioBackend instead of the concrete PipeWireBackend.
- lib: gate the unix-only 0o600 log-file mode code (+ its test); Windows
  logs inherit the directory ACL.
- audio_probe: gate this PipeWire diagnostic to unix with a stub main.
- app: open URLs via rundll32 on windows, xdg-open on unix (shell-free).
- ci: add .gitea/workflows/windows-build.yml (M1) — build + lib tests for
  x86_64-pc-windows-msvc, with CMAKE_POLICY_VERSION_MINIMUM=3.5 for the
  vendored libopus build. Needs a windows act_runner to actually run.

Linux build/clippy/tests green (316/316). The Windows path is verified by
inspection only (no local Windows toolchain); CI is the real gate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.toml

@@ -30,17 +30,12 @@ bytes = "1.11.1"
 dirs = "6.0.0"
 iced = { version = "0.14.0", features = ["canvas", "image"] }
 # W4 custom avatars: decode/resize an arbitrary user image (png/jpeg only to keep
-# the codec surface small) and a native file picker (xdg-portal backend, no GTK).
+# the codec surface small). The matching native file picker (`rfd`) is platform-
+# gated below — its backend differs per OS (xdg-portal on Linux, Win32 on Windows).
 image = { version = "0.25", default-features = false, features = ["png", "jpeg"] }
-rfd = { version = "0.17", default-features = false, features = ["xdg-portal"] }
 iroh = "1.0.0-rc.0"
 iroh-gossip = "0.99.0"
 opus = "0.3.1"
-# v0_3_49 exposes `Buffer::requested()` (the graph's per-cycle quantum), used by
-# the playback RT callback to fill exactly what the device asks for instead of
-# pinning the buffer to a hard-coded 1024-frame quantum (crackle on non-1024
-# hardware). The field has existed in libpipewire since 0.3.49 (2022).
-pipewire = { version = "0.9", features = ["v0_3_49"] }
 rand = "0.10.1"
 ringbuf = "0.5.0"
 serde = { version = "1.0.228", features = ["derive"] }
@@ -48,3 +43,24 @@ serde_json = "1.0.150"
 thiserror = "2.0.18"
 tokio = { version = "1.52.3", features = ["full"] }
 tokio-stream = "0.1.18"
+
+# --- Platform-specific dependencies -----------------------------------------
+# Audio and the native file-picker backends differ per OS. Everything else in the
+# app talks to the `AudioBackend` trait and the `PlatformAudioBackend` alias (see
+# `src/audio/mod.rs`), so platform selection is confined to these few lines.
+
+[target.'cfg(unix)'.dependencies]
+# Linux audio backend. v0_3_49 exposes `Buffer::requested()` (the graph's per-cycle
+# quantum), used by the playback RT callback to fill exactly what the device asks
+# for instead of a hard-coded 1024-frame quantum (crackle on non-1024 hardware).
+# The field has existed in libpipewire since 0.3.49 (2022).
+pipewire = { version = "0.9", features = ["v0_3_49"] }
+# Native file picker via the XDG desktop portal (no GTK) on Linux.
+rfd = { version = "0.17", default-features = false, features = ["xdg-portal"] }
+
+[target.'cfg(windows)'.dependencies]
+# Native file picker using the built-in Win32 dialog backend on Windows.
+rfd = { version = "0.17", default-features = false }
+# NOTE: the Windows audio backend (cpal/WASAPI) lands in Phase 1. Until then the
+# Windows build uses the no-op `CpalBackend` stub in `src/audio/cpal_impl.rs`,
+# which needs no extra dependency.