fix(git): surface non-git-repo as a typed error and align all panels via workspace state

Consolidate `.git` presence detection into a shared `git_repo` module used by both the workspace state watcher and the command preflight helper, replacing duplicated local definitions.

Introduce `AppErrorCode::NotAGitRepository` (HTTP 422) and preflight eleven frontend-callable git commands (log, status, list-branches, diff, diff-with-branch, show-diff, show-file, push-info, list-remotes, list-all-branches, commit-branches) so non-git folders short-circuit with a structured error instead of leaking locale-dependent git stderr.

Frontend `isNotAGitRepoError` checks the error code first and falls back to a multi-language regex list centralized in `src/i18n/git-error-patterns.ts`, covering the nine languages git actually translates into.

Wire the git log panel to `workspaceState.isGitRepo` rather than a local cached flag, so running `git init` or deleting `.git` externally propagates through the watcher and refreshes the panel automatically.

src-tauri/src/git_repo.rs

@@ -0,0 +1,40 @@
+//! Single source of truth for "is this path a git repository?" detection.
+//!
+//! The check is deliberately strict: the exact path must contain a `.git`
+//! entry (directory for regular repos, file for linked worktrees and
+//! submodules). We do **not** walk up to ancestors.
+//!
+//! Rationale: codeg scopes every workspace-facing feature (file tree
+//! watcher, git changes panel, log panel) to the directory the user opens.
+//! If one code path walks up and another doesn't, the UI falls into a
+//! "schizophrenic" state where some panels see a repo and others don't.
+//! Keeping the primitive strict forces every consumer onto the same
+//! interpretation.
+//!
+//! Bare repositories are intentionally not supported — they have no working
+//! tree, which makes them an unusual target for a workspace-oriented editor.
+
+use std::path::Path;
+
+use crate::app_error::AppCommandError;
+
+/// Returns true when `path` is the root of a git working tree.
+///
+/// `.git` may be a directory (normal repo) or a file (worktree/submodule
+/// pointer). `Path::exists` treats both as present.
+pub fn is_git_repo(path: &Path) -> bool {
+    path.join(".git").exists()
+}
+
+/// Preflight guard for git commands. Short-circuits with a typed error code
+/// when the target path is not a git working tree, so callers avoid locale-
+/// dependent stderr parsing for the most common "wrong folder" failure.
+pub fn ensure_git_repo(path: &str) -> Result<(), AppCommandError> {
+    if is_git_repo(Path::new(path)) {
+        Ok(())
+    } else {
+        Err(AppCommandError::not_a_git_repository(format!(
+            "Not a Git repository: {path}"
+        )))
+    }
+}