feat(integrations): add review fetching and Linux keyring support

Add a cross-provider Review Center and improve credential handling.
Backend integrations fetch and normalize PRs from GitHub, GitLab,
Gitea, and Azure DevOps with improved timeouts and parsing. Credentials
now use a global lock and support secret-tool on Linux to avoid races.

- Normalize review data across providers (GitHub/GitLab/Gitea/Azure)
- Serialize credential access with OnceLock and use secret-tool on Linux
- Add frontend ReviewCenter component and related UI updates

src/lib/RepoTabs.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { Folder, GitBranch, Plus, X } from "@lucide/svelte";
+  import { Folder, GitBranch, GitPullRequest, Plus, X } from "@lucide/svelte";
 
   interface RepositoryTabItem {
     path: string;
@@ -7,11 +7,12 @@
     branch: string | null;
   }
 
-  export let activeView: "management" | "repository" = "management";
+  export let activeView: "management" | "review-center" | "repository" = "management";
   export let repoTabs: RepositoryTabItem[] = [];
   export let isBusy: boolean = false;
   export let language: "en" | "de" = "en";
   export let onOpenManagement: () => void = () => {};
+  export let onOpenReviewCenter: () => void = () => {};
   export let isActive: (path: string) => boolean = () => false;
   export let onSelect: (path: string) => void | Promise<void> = () => {};
   export let onClose: (path: string, event: MouseEvent) => void | Promise<void> = () => {};
@@ -33,6 +34,12 @@
   </button>
 
   <div class="repo-tabs-scroll">
+    <div class="repo-tab-wrap review-center-tab" class:active={activeView === "review-center"} role="presentation">
+      <button class="repo-tab" type="button" onclick={onOpenReviewCenter} disabled={isBusy} title="Review Center" aria-label="Review Center">
+        <GitPullRequest size={13} aria-hidden="true" />
+        <span>PR</span>
+      </button>
+    </div>
     {#each repoTabs as repo (repo.path)}
       <div
         class="repo-tab-wrap"