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.

.claude/settings.local.json

@@ -58,7 +58,20 @@
       "Bash(: *)",
       "Bash(exit 0 *)",
       "Bash(rustc -O sanitize_test.rs -o sanitize_test)",
-      "Bash(./sanitize_test)"
+      "Bash(./sanitize_test)",
+      "Bash(echo \"exit:$?\")",
+      "Bash(grep -rlP \"[äöüßÄÖÜ]\" src src-tauri/src src-tauri/crates --include=\"*.rs\" --include=\"*.svelte\" --include=\"*.ts\")",
+      "Bash(echo \"---exit $?---\")",
+      "Bash(apt-cache policy *)",
+      "Bash(timeout 5 curl -sI http://archive.ubuntu.com)",
+      "Bash(sudo -n apt-get install -y libdbus-1-dev pkg-config)",
+      "Bash(grep -n '\"Unerwarteter Git-Log-Eintrag: {}\",' src-tauri/src/git.rs)",
+      "Bash(grep -E \"git_lite$|tauri_git_lite$\")",
+      "Bash(rustfmt --edition 2024 --check src-tauri/src/git.rs)",
+      "Bash(echo \"EXIT:$?\")",
+      "Bash(ls target/)",
+      "Bash(rustup target *)",
+      "Bash(echo \"exit code: $?\")"
     ]
   }
 }