fix: commit & push now saves pending content and refreshes modified files

Root cause: Two problems caused "0 files changed":
1. loadModifiedFiles() was only called on mount, never refreshed after saves
2. Pending editor content wasn't flushed to disk before git commit

Fix:
- useEditorSave: add savePending() to flush unsaved content, onAfterSave
  callback to refresh git status after Cmd+S
- useCommitFlow: new hook managing save→commit→push flow with proper
  sequencing (save pending → refresh files → show dialog → commit)
- App.tsx: wire onAfterSave to loadModifiedFiles, use useCommitFlow
- git.rs: include stdout in error when stderr is empty (fixes "nothing
  to commit" message being swallowed)
- mock-tauri: track saved files so get_modified_files reflects edits

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/hooks/useCommitFlow.ts

@@ -0,0 +1,36 @@
+import { useCallback, useState } from 'react'
+
+interface CommitFlowConfig {
+  savePending: () => Promise<void>
+  loadModifiedFiles: () => Promise<void>
+  commitAndPush: (message: string) => Promise<string>
+  setToastMessage: (msg: string | null) => void
+}
+
+/** Manages the Commit & Push dialog state and the save→commit→push flow. */
+export function useCommitFlow({ savePending, loadModifiedFiles, commitAndPush, setToastMessage }: CommitFlowConfig) {
+  const [showCommitDialog, setShowCommitDialog] = useState(false)
+
+  const openCommitDialog = useCallback(async () => {
+    await savePending()
+    await loadModifiedFiles()
+    setShowCommitDialog(true)
+  }, [savePending, loadModifiedFiles])
+
+  const handleCommitPush = useCallback(async (message: string) => {
+    setShowCommitDialog(false)
+    try {
+      await savePending()
+      const result = await commitAndPush(message)
+      setToastMessage(result)
+      loadModifiedFiles()
+    } catch (err) {
+      console.error('Commit failed:', err)
+      setToastMessage(`Commit failed: ${err}`)
+    }
+  }, [savePending, commitAndPush, loadModifiedFiles, setToastMessage])
+
+  const closeCommitDialog = useCallback(() => setShowCommitDialog(false), [])
+
+  return { showCommitDialog, openCommitDialog, handleCommitPush, closeCommitDialog }
+}