Move the snap to core24 and replace the unity7 plug

CLAUDE.md

@@ -45,7 +45,11 @@ Everything happens in `main()` in `main.go`, in a straight-line pipeline:
 
 ### Temp file / Snap sandboxing (`getTempDir`, `isSnap`)
 
-Output location resolution order: `MDVIEW_DIR` env var → (if running as a Snap, per `SNAP_USER_COMMON` env var) `~/mdview-temp` → OS default temp dir. `MDVIEW_DIR` and `~/mdview-temp` are created with `MkdirAll`. Before writing a temp file, `removeStaleTempFiles` deletes generated `mdview<32 hex>.html` files older than 24 hours from that directory (the browser reads the new file after mdview exits, so mdview cannot delete it right away). This exists because Snap-sandboxed browsers (e.g. Snap Firefox) can't read from `/tmp`, only from the user's home directory — `check()` writes errors to stderr, names the failing path, and prints a Snap-specific hint for permission and not-found errors for this reason.
+Output location resolution order: `MDVIEW_DIR` env var → (if running as a Snap, per `SNAP_USER_COMMON` env var) `~/mdview-temp` (the home directory comes from `SNAP_REAL_HOME`, because snapd remaps `HOME` to the snap's private data directory) → OS default temp dir. `MDVIEW_DIR` and `~/mdview-temp` are created with `MkdirAll`. Before writing a temp file, `removeStaleTempFiles` deletes generated `mdview<32 hex>.html` files older than 24 hours from that directory (the browser reads the new file after mdview exits, so mdview cannot delete it right away). This exists because Snap-sandboxed browsers (e.g. Snap Firefox) can't read from `/tmp`, only from the user's home directory — `check()` writes errors to stderr, names the failing path, and prints a Snap-specific hint for permission and not-found errors for this reason.
+
+The snap (`snap/snapcraft.yaml`, `base: core24`, `platforms:` instead of `architectures:`) plugs `home` and
+`desktop`. `desktop` (auto-connected) lets `xdg-open` launch the default browser; it replaced the transitional
+`unity7` plug.
 
 ### Vendored `mermaid.min.js`
 

main.go

@@ -243,7 +243,12 @@ func getTempDir() string {
 	}
 
 	if isSnap() {
-		var tmpdir = filepath.Join(os.Getenv("HOME"), "mdview-temp")
+		// snapd points HOME at the snap's private data directory, so use the real home directory
+		home := os.Getenv("SNAP_REAL_HOME")
+		if home == "" {
+			home = os.Getenv("HOME")
+		}
+		var tmpdir = filepath.Join(home, "mdview-temp")
 		check(os.MkdirAll(tmpdir, 0700))
 		return tmpdir
 	}

main_test.go

@@ -325,6 +325,19 @@ func TestGetTempDir(t *testing.T) {
 		home := t.TempDir()
 		t.Setenv("MDVIEW_DIR", "")
 		t.Setenv("SNAP_USER_COMMON", "/snap/common")
+		t.Setenv("HOME", filepath.Join(home, "snap", "mdview", "1"))
+		t.Setenv("SNAP_REAL_HOME", home)
+		want := filepath.Join(home, "mdview-temp")
+		if got := getTempDir(); got != want {
+			t.Fatalf("getTempDir = %q, want %q", got, want)
+		}
+	})
+
+	t.Run("snap without SNAP_REAL_HOME falls back to HOME", func(t *testing.T) {
+		home := t.TempDir()
+		t.Setenv("MDVIEW_DIR", "")
+		t.Setenv("SNAP_USER_COMMON", "/snap/common")
+		t.Setenv("SNAP_REAL_HOME", "")
 		t.Setenv("HOME", home)
 		want := filepath.Join(home, "mdview-temp")
 		if got := getTempDir(); got != want {

snap/snapcraft.yaml

@@ -7,13 +7,11 @@ description: |
   Type `mdview --help` for options. https://github.com/mapitman/mdview
 grade: stable
 confinement: strict
-base: core22
+base: core24
 apps:
     mdview:
       command: bin/mdview
-      plugs: [home,unity7]
-      environment:
-        HOME: /home/$USER
+      plugs: [home, desktop]
 adopt-info: mdview
 parts:
   mdview:
@@ -29,10 +27,7 @@ parts:
     #   - libblkid-dev
     build-snaps:
       - go
-architectures:
-  - build-on: [amd64]
-    build-for: [amd64]
-  - build-on: [arm64]
-    build-for: [arm64]
-  - build-on: [armhf]
-    build-for: [armhf]
+platforms:
+  amd64:
+  arm64:
+  armhf: