fix(library): keep the resolved env fallback on failure statuses

The backend has no built-in library default — _get_dlc_dir() returns None
when DLC_DIR is unset and config.json is unusable — so deleting DLC_DIR on
invalid-config/write-failed stranded the user with an empty library and
only a console warn (corrupt config.json, or a saved dlc_dir pointing at
an unplugged drive, previously still produced a working library via the
env fallback).

invalid-config and write-failed now export the resolved fallback as
DLC_DIR, restoring pre-#117 behaviour exactly and only where config.json
cannot own the path. Config-owned dynamic refresh is unchanged for the
configured/bootstrapped paths.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main/library-path-config.ts

@@ -56,6 +56,12 @@ export function applyLibraryPathToPythonEnvironment(
  * Normal desktop launches instead keep the selected path in config.json so
  * the backend can re-read a Settings change on the next manual scan without
  * requiring a process restart.
+ *
+ * Failure statuses (invalid-config, write-failed) still export the resolved
+ * fallback as DLC_DIR: the backend has NO built-in default (_get_dlc_dir
+ * returns None), so removing the env var when config.json cannot own the
+ * path would strand the user with an empty library and only a console warn.
+ * Config-owned dynamic behaviour applies exactly when config.json is usable.
  */
 export function prepareLibraryPathForPython(
     configDir: string,
@@ -79,6 +85,7 @@ export function prepareLibraryPathForPython(
             if (!isPlainObject(parsed)) {
                 return {
                     status: 'invalid-config',
+                    environmentDlcDir: resolvedDlcDir,
                     error: 'config.json is not a JSON object',
                 };
             }
@@ -86,6 +93,7 @@ export function prepareLibraryPathForPython(
         } catch (err) {
             return {
                 status: 'invalid-config',
+                environmentDlcDir: resolvedDlcDir,
                 error: err instanceof Error ? err.message : String(err),
             };
         }
@@ -97,12 +105,14 @@ export function prepareLibraryPathForPython(
             }
             return {
                 status: 'invalid-config',
+                environmentDlcDir: resolvedDlcDir,
                 error: 'config.json dlc_dir is not an existing directory',
             };
         }
         if (configured !== undefined && configured !== null && configured !== '') {
             return {
                 status: 'invalid-config',
+                environmentDlcDir: resolvedDlcDir,
                 error: 'config.json dlc_dir is not a string',
             };
         }
@@ -120,6 +130,7 @@ export function prepareLibraryPathForPython(
         } catch { /* best-effort temporary-file cleanup */ }
         return {
             status: 'write-failed',
+            environmentDlcDir: resolvedDlcDir,
             error: err instanceof Error ? err.message : String(err),
         };
     }

tests/library-path-config.test.js

@@ -134,6 +134,49 @@ test('a corrupt config is never overwritten during bootstrap', () => {
     assert.equal(fs.readFileSync(configFile, 'utf8'), '{broken');
 });
 
+test('invalid-config exports the resolved fallback so the backend keeps a library', () => {
+    // The backend has no built-in default (_get_dlc_dir returns None): with
+    // the env var deleted AND config.json unusable, scans and starter
+    // seeding would silently skip. Failure statuses must keep the pre-#117
+    // env fallback.
+    const configDir = tmpConfigDir();
+    const configFile = path.join(configDir, 'config.json');
+    fs.writeFileSync(configFile, JSON.stringify({ dlc_dir: path.join(configDir, 'gone') }));
+
+    const result = prepareLibraryPathForPython(configDir, 'C:\\Default Songs');
+
+    assert.equal(result.status, 'invalid-config');
+    assert.equal(result.environmentDlcDir, 'C:\\Default Songs');
+
+    const environment = {};
+    applyLibraryPathToPythonEnvironment(environment, result);
+    assert.deepEqual(environment, { DLC_DIR: 'C:\\Default Songs' });
+});
+
+test('corrupt config exports the resolved fallback without being overwritten', () => {
+    const configDir = tmpConfigDir();
+    const configFile = path.join(configDir, 'config.json');
+    fs.writeFileSync(configFile, '{broken');
+
+    const result = prepareLibraryPathForPython(configDir, 'C:\\Default Songs');
+
+    assert.equal(result.status, 'invalid-config');
+    assert.equal(result.environmentDlcDir, 'C:\\Default Songs');
+    assert.equal(fs.readFileSync(configFile, 'utf8'), '{broken');
+});
+
+test('write-failed exports the resolved fallback so the backend keeps a library', () => {
+    // Force the config write to fail by making config.json's parent a file.
+    const root = tmpConfigDir();
+    const configDir = path.join(root, 'not-a-dir');
+    fs.writeFileSync(configDir, 'x');
+
+    const result = prepareLibraryPathForPython(configDir, 'C:\\Default Songs');
+
+    assert.equal(result.status, 'write-failed');
+    assert.equal(result.environmentDlcDir, 'C:\\Default Songs');
+});
+
 test('Python environment omits DLC_DIR when config owns the library path', () => {
     const environment = { DLC_DIR: 'C:\\Stale Parent Value', KEEP: 'yes' };