fix: list and open sessions across project directories

Sessions outside the server's launch directory were invisible and returned
404 on open, leaving the model picker empty. List without the SDK's
process-cwd filter and normalize working directories at the API boundary and
when reading stored session data, tolerating separator/normalization
differences (including Windows backslash vs forward slash). Requires Pi
coding agent SDK 0.78.0 or newer.

src/server/sessions/oauthLoginFlowService.test.ts

@@ -45,9 +45,7 @@ describe("OAuthLoginFlowService", () => {
       providerId: "test-provider",
       providerName: "Test Provider",
       authStorage: fakeAuthStorage(async (_providerId, callbacks) => {
-        const select = callbacks.onSelect;
-        if (select === undefined) throw new Error("Expected select callback");
-        selectedValue = await select({
+        selectedValue = await callbacks.onSelect({
           message: "Choose account",
           options: [{ id: "work", label: "Work" }, { id: "personal", label: "Personal" }],
         });