feat(ui): localize commit AI and git error messages to English

Translate commit AI prompts, model labels, and git/keychain errors to
English so the app and generated messages are consistent. Also add a
discard confirmation dialog and update the UI text/styles to match the
new flow.

- src-tauri/crates/commit_ai/src/cloud.rs
  - Translate HTTP and API error messages to English.
  - Keep request timeout and token sizing behavior unchanged.
- src-tauri/crates/commit_ai/src/lib.rs
  - Translate model labels, prompt text, and validation errors.
  - Keep diff truncation and message sanitization logic intact.
- src-tauri/src/git.rs
  - Translate git, credential, merge, and history errors.
  - Update AI provider validation messages to English.
- src/lib/components/*
  - Update AI settings, commit panel, credential, and loading UI text.
  - Add discard confirmation dialog for destructive actions.
- src/App.svelte, src/app.css
  - Adjust app layout and styling for the new dialog and text changes.

src/lib/components/DiscardConfirmDialog.svelte

@@ -0,0 +1,74 @@
+<script lang="ts">
+  import { AlertTriangle, LoaderCircle, RotateCcw, X } from "@lucide/svelte";
+  import type { GitFileStatus } from "../types";
+
+  interface Props {
+    file: GitFileStatus;
+    staged: boolean;
+    scope: "file" | "hunk";
+    isBusy: boolean;
+    onConfirm: () => void | Promise<void>;
+    onClose: () => void;
+  }
+
+  let {
+    file,
+    staged = false,
+    scope = "file",
+    isBusy = false,
+    onConfirm = () => {},
+    onClose = () => {},
+  }: Props = $props();
+
+  let targetPath = $derived(file.old_path ? `${file.old_path} -> ${file.path}` : file.path);
+  let title = $derived(scope === "hunk" ? "Discard hunk?" : "Discard file changes?");
+  let scopeLabel = $derived(scope === "hunk" ? "Selected hunk" : "File changes");
+  let sourceLabel = $derived(staged ? "staged changes" : "unstaged changes");
+
+  function closeFromBackdrop(event: MouseEvent) {
+    if (isBusy || event.target !== event.currentTarget) return;
+    onClose();
+  }
+</script>
+
+<div class="dialog-backdrop" role="presentation" onclick={closeFromBackdrop}>
+  <div class="dialog discard-confirm-dialog" role="dialog" aria-modal="true" aria-label={title}>
+    <header class="dialog-header">
+      <div>
+        <span class="eyebrow">Confirm discard</span>
+        <p class="dialog-title">{title}</p>
+      </div>
+      <button class="btn-sm dialog-close" type="button" onclick={onClose} disabled={isBusy} aria-label="Close">
+        <X size={16} aria-hidden="true" />
+      </button>
+    </header>
+
+    <div class="discard-confirm-body">
+      <div class="discard-warning-icon" aria-hidden="true">
+        <AlertTriangle size={22} />
+      </div>
+
+      <div class="discard-confirm-copy">
+        <p>
+          This will reset the {sourceLabel} for the {scopeLabel.toLowerCase()} below.
+        </p>
+        <code class="discard-target" title={targetPath}>{targetPath}</code>
+        <p class="discard-warning-text">
+          This cannot be undone. If the file only exists in your working tree, it can be deleted entirely.
+        </p>
+      </div>
+    </div>
+
+    <footer class="discard-confirm-actions">
+      <button class="btn-secondary" type="button" onclick={onClose} disabled={isBusy}>Cancel</button>
+      <button class="btn-danger" type="button" onclick={onConfirm} disabled={isBusy}>
+        {#if isBusy}
+          <LoaderCircle class="spin" size={15} aria-hidden="true" />
+        {:else}
+          <RotateCcw size={15} aria-hidden="true" />
+        {/if}
+        Discard
+      </button>
+    </footer>
+  </div>
+</div>