fix(desktop): put the checkout on the source backend's PYTHONPATH

A developer/E2E interpreter override need not have the checkout installed,
and the backend runs in the user's workspace cwd, so `-m hermes_cli.main`
only resolved from an editable .venv. The Desktop core E2E on the PM test
environment died with "No module named hermes_cli" on every local boot.
Name this checkout explicitly; the scrub of an inherited value stays.

apps/desktop/electron/source-backend.test.ts

@@ -183,8 +183,9 @@ test.skipIf(process.platform === 'win32')(
           assert.ok(source)
           assert.equal(source.command, fixture.python)
           assert.match(
+            // The real spawn runs in the user's workspace, never the checkout.
             execFileSync(source.command, source.args, {
-              cwd: source.root,
+              cwd: temp,
               env: { ...env, ...source.env },
               encoding: 'utf8',
               timeout: 15_000
@@ -224,7 +225,7 @@ test('Windows console selection uses only the selected interpreter directory', (
     fs.writeFileSync(consolePython, '')
     const backend: SourceBackend | null = createSourcePythonBackend(root, selected, ['serve'], { isWindows: true })
     assert.equal(backend?.command, consolePython)
-    assert.equal(backend?.env.PYTHONPATH, '')
+    assert.equal(backend?.env.PYTHONPATH, root)
     assert.equal(backend?.env.PYTHONHOME, '')
     assert.equal(createSourcePythonBackend(root, selected, [], { isWindows: false })?.command, selected)
     assert.equal(createSourcePythonBackend(root, null, []), null)

apps/desktop/electron/source-backend.ts

@@ -96,7 +96,10 @@ export function createSourcePythonBackend(
     label: `Hermes source at ${root}`,
     command,
     args: ['-m', 'hermes_cli.main', ...args],
-    env: buildDesktopBackendEnv({ currentEnv: options.env ?? process.env }),
+    // The backend runs in the user's workspace cwd, and the selected
+    // interpreter need not have this checkout installed: name it explicitly.
+    // (The scrubbed inherited value could point at another checkout.)
+    env: { ...buildDesktopBackendEnv({ currentEnv: options.env ?? process.env }), PYTHONPATH: root },
     root,
     bootstrap: false,
     shell: false,