refactor: git overhaul with git2 lib

  - replace Git Pane status and diff loading with git2-backed Rust commands
  - split Git Pane data flow into summary loading and per-file diff loading
  - move diff parsing into Rust and return structured rows to the frontend
  - lazily load file diffs, cache detail results across collapse and reopen, and avoid preloading large diffs
  - auto-expand only the first 10 files per section and keep large diffs collapsed by default
  - keep binary files in staged and unstaged lists while rendering them as placeholder-only cards
  - synthesize diffs for untracked text files so additions, line numbers, and content render correctly
  - disable syntax highlighting for large diffs and plain-text or data extensions
  - preserve git commit hooks, signing, and amend semantics by delegating commit operations to the git CLI
  - fix rename diff loading and unstage-all behavior so old and new paths stay in sync
  - add regression coverage for untracked diffs, rename detail handling, binary visibility, and rename unstage behavior

Author: brucehsu

apps/desktop/src-tauri/Cargo.lock

@@ -12,6 +12,7 @@ tauri-plugin-dialog = "2"
 tauri-plugin-notification = "2"
 portable-pty = "0.8"
 base64 = "0.22"
+git2 = "0.20"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 

apps/desktop/src-tauri/Cargo.toml

@@ -9,7 +9,8 @@ commands.allow = [
   "resolve_repo_root",
   "get_git_branch",
   "rename_git_branch",
-  "get_git_diff",
+  "get_git_change_summary",
+  "get_git_file_diff",
   "stage_all_changes",
   "unstage_all_changes",
   "discard_all_changes",
@@ -39,7 +40,8 @@ commands.allow = [
   "resolve_repo_root",
   "get_git_branch",
   "rename_git_branch",
-  "get_git_diff",
+  "get_git_change_summary",
+  "get_git_file_diff",
   "stage_all_changes",
   "unstage_all_changes",
   "discard_all_changes",