feat(git): add opt-in for unrelated histories during pull

Allow pulling repositories with unrelated commit histories when the user
explicitly opts in. The pull argument construction was refactored and
branch resolution made more robust so the backend can include the
--allow-unrelated-histories flag when requested.

- Extract pull argument logic and add support for allowing unrelated histories.
- Prompt users in the UI to confirm merging separate histories and retry pull.
- Restyle and improve the update toast UI for better layout and responsiveness.

src/lib/components/UpdateToast.svelte

@@ -53,7 +53,7 @@
   );
   const versionLabel = $derived(
     version && currentVersion
-      ? `${currentVersion} -> ${version}`
+      ? `${currentVersion} → ${version}`
       : version
         ? `Version ${version}`
         : "New version",
@@ -69,34 +69,32 @@
   role={state === "error" ? "alert" : "status"}
   aria-live={state === "error" ? "assertive" : "polite"}
 >
-  <div class="update-toast-glow" aria-hidden="true"></div>
-
-  <div class="update-toast-icon" class:busy={isBusy}>
-    {#if state === "downloading"}
-      <LoaderCircle class="spin" size={21} aria-hidden="true" />
-    {:else if state === "installed"}
-      <PackageCheck size={21} aria-hidden="true" />
-    {:else if state === "error"}
-      <AlertCircle size={21} aria-hidden="true" />
-    {:else}
-      <Sparkles size={21} aria-hidden="true" />
-    {/if}
-  </div>
-
-  <div class="update-toast-content">
-    <div class="update-toast-top">
-      <div class="update-toast-copy">
-        <span class="update-toast-kicker">{versionLabel}</span>
-        <h2>{title}</h2>
-      </div>
-
-      {#if !isBusy}
-        <button class="update-toast-close" type="button" onclick={onDismiss} title="Close" aria-label="Close">
-          <X size={15} aria-hidden="true" />
-        </button>
+  <header class="update-toast-header">
+    <div class="update-toast-icon" class:busy={isBusy}>
+      {#if state === "downloading"}
+        <LoaderCircle class="spin" size={18} aria-hidden="true" />
+      {:else if state === "installed"}
+        <PackageCheck size={18} aria-hidden="true" />
+      {:else if state === "error"}
+        <AlertCircle size={18} aria-hidden="true" />
+      {:else}
+        <Sparkles size={18} aria-hidden="true" />
       {/if}
     </div>
 
+    <div class="update-toast-copy">
+      <span class="update-toast-kicker">Gitty update · {versionLabel}</span>
+      <h2>{title}</h2>
+    </div>
+
+    {#if !isBusy}
+      <button class="update-toast-close" type="button" onclick={onDismiss} title="Close" aria-label="Close">
+        <X size={15} aria-hidden="true" />
+      </button>
+    {/if}
+  </header>
+
+  <div class="update-toast-body">
     <p>{description}</p>
 
     {#if showProgress}
@@ -115,33 +113,34 @@
       </div>
     {/if}
 
-    <div class="update-toast-actions">
-      {#if state === "installed"}
-        <button class="update-toast-secondary" type="button" onclick={onDismiss}>
-          Got it
-        </button>
-      {:else if state === "error"}
-        <button class="update-toast-secondary" type="button" onclick={onDismiss}>
-          Close
-        </button>
-        <button class="update-toast-primary" type="button" onclick={onInstall}>
-          <Download size={14} aria-hidden="true" />
-          Try again
-        </button>
-      {:else}
-        <button class="update-toast-secondary" type="button" onclick={onLater} disabled={isBusy}>
-          Later
-        </button>
-        <button class="update-toast-primary" type="button" onclick={onInstall} disabled={isBusy}>
-          {#if isBusy}
-            <LoaderCircle class="spin" size={14} aria-hidden="true" />
-            Installing
-          {:else}
-            <Download size={14} aria-hidden="true" />
-            Install
-          {/if}
-        </button>
-      {/if}
-    </div>
   </div>
+
+  <footer class="update-toast-actions">
+    {#if state === "installed"}
+      <button class="update-toast-secondary" type="button" onclick={onDismiss}>
+        Got it
+      </button>
+    {:else if state === "error"}
+      <button class="update-toast-secondary" type="button" onclick={onDismiss}>
+        Close
+      </button>
+      <button class="update-toast-primary" type="button" onclick={onInstall}>
+        <Download size={14} aria-hidden="true" />
+        Try again
+      </button>
+    {:else}
+      <button class="update-toast-secondary" type="button" onclick={onLater} disabled={isBusy}>
+        Later
+      </button>
+      <button class="update-toast-primary" type="button" onclick={onInstall} disabled={isBusy}>
+        {#if isBusy}
+          <LoaderCircle class="spin" size={14} aria-hidden="true" />
+          Installing
+        {:else}
+          <Download size={14} aria-hidden="true" />
+          Install
+        {/if}
+      </button>
+    {/if}
+  </footer>
 </section>