feat(shortcuts): Add Ctrl/Cmd+1..4 to switch main views

Add keyboard shortcuts (Ctrl/Cmd + 1–4) to jump to the four main views:
Dashboard, Repositories, Pull Requests and Issues & Boards.

- Handler runs in the capture phase so editors/dialogs can still trap keys.
- Switching is blocked when the app is busy or any modal/overlay is open; repeats
  and combinations with Alt/Shift are ignored.
- When switching to the repository view the shortcut selects an existing repo
  tab (it will not open a native file dialog if no repo is available).

Also update tab button titles to surface the shortcuts and add entries to the
help overlay (English and German).

src/lib/RepoTabs.svelte

@@ -89,10 +89,10 @@
 
 <header class="workspace-navigation">
   <nav class="global-navigation" aria-label={language === "de" ? "Hauptnavigation" : "Main navigation"}>
-    <button type="button" class:active={activeView === "management"} aria-current={activeView === "management" ? "page" : undefined} disabled={isBusy} onclick={onOpenManagement}><House size={17}/><span>Dashboard</span></button>
-    <button type="button" class:active={activeView === "repository"} aria-current={activeView === "repository" ? "page" : undefined} disabled={isBusy} onclick={onOpenRepositories}><Database size={17}/><span>Repositories</span></button>
-    <button type="button" class:active={activeView === "review-center"} aria-current={activeView === "review-center" ? "page" : undefined} disabled={isBusy} onclick={onOpenReviewCenter}><GitPullRequest size={17}/><span>Pull Requests</span></button>
-    <button type="button" class:active={activeView === "issues"} aria-current={activeView === "issues" ? "page" : undefined} disabled={isBusy} onclick={onOpenIssues}><Columns3 size={17}/><span>Issues &amp; Boards</span></button>
+    <button type="button" class:active={activeView === "management"} aria-current={activeView === "management" ? "page" : undefined} disabled={isBusy} onclick={onOpenManagement} title="Dashboard (Ctrl + 1)"><House size={17}/><span>Dashboard</span></button>
+    <button type="button" class:active={activeView === "repository"} aria-current={activeView === "repository" ? "page" : undefined} disabled={isBusy} onclick={onOpenRepositories} title="Repositories (Ctrl + 2)"><Database size={17}/><span>Repositories</span></button>
+    <button type="button" class:active={activeView === "review-center"} aria-current={activeView === "review-center" ? "page" : undefined} disabled={isBusy} onclick={onOpenReviewCenter} title="Pull Requests (Ctrl + 3)"><GitPullRequest size={17}/><span>Pull Requests</span></button>
+    <button type="button" class:active={activeView === "issues"} aria-current={activeView === "issues" ? "page" : undefined} disabled={isBusy} onclick={onOpenIssues} title="Issues & Boards (Ctrl + 4)"><Columns3 size={17}/><span>Issues &amp; Boards</span></button>
   </nav>
   {#if activeView === "repository"}
     <div class="repository-row">

src/lib/components/HelpOverlay.svelte

@@ -249,6 +249,7 @@
           summary: "Die Hilfe ist überall erreichbar. Dialoge lassen sich konsistent schließen und Suchfelder direkt fokussieren.",
           commands: [
             { command: "Ctrl + /", description: "Diese Hilfe öffnen" },
+            { command: "Ctrl + 1 … 4", description: "Zwischen Dashboard, Repositories, Pull Requests und Issues & Boards wechseln" },
             { command: "Ctrl + A", description: "Alle Dateien in der aktiven Statusliste („Ungestaged“ oder „Gestaged“) auswählen" },
             { command: "Escape", description: "Aktuelles Overlay oder Dialogfenster schließen – in der Statusliste die aktuelle Auswahl aufheben" },
             { command: "Tab / Shift + Tab", description: "Zwischen Bedienelementen wechseln" },
@@ -460,6 +461,7 @@
           summary: "Help is available everywhere. Dialogs close consistently and search fields receive focus automatically.",
           commands: [
             { command: "Ctrl + /", description: "Open this help center" },
+            { command: "Ctrl + 1 … 4", description: "Switch between Dashboard, Repositories, Pull Requests and Issues & Boards" },
             { command: "Ctrl + A", description: "Select every file in the focused status list (Unstaged or Staged)" },
             { command: "Escape", description: "Close the current overlay or dialog – in the status list, clear the current selection" },
             { command: "Tab / Shift + Tab", description: "Move between controls" },