macOS desktop: install + in-app self-update (#35607)

* fix(installer): align macOS HERMES_HOME with the rest of the stack

paths.rs computed the macOS Hermes home as ~/Library/Application Support/
hermes, but nothing else does: hermes_constants.get_hermes_home() (Python),
scripts/install.sh, and the Electron desktop's resolveHermesHome() all use
~/.hermes on macOS. The drift meant the Tauri installer wrote the install to
one directory and the desktop looked for it in another, so a fresh GUI
install never found its backend (the file's own comment warned this exact
drift would break things). Use ~/.hermes on macOS to match.

* fix(install.sh): always emit a stage result frame on failure

Stage helpers (clone_repo, install_deps, check_python, …) were written for
the monolithic flow and call `exit 1` on failure. Under `--stage`, that
terminated the process before the JSON result frame was printed, so the
installer's parse_stage_result saw "no frame" instead of a clean
{ok:false,...} contract response. Run the stage body in a subshell so an
`exit` only unwinds the subshell and the parent still emits the frame.

* feat(install.sh): auto-provision git on macOS/Linux (parity with install.ps1)

install.ps1 downloads PortableGit on Windows, but install.sh just printed a
"please install git" hint and exited — so a fresh Mac with no developer tools
(no Xcode CLT → no git) couldn't get past the clone step. check_git now tries
to install git before bailing:
  - macOS: Homebrew if present (headless), else `xcode-select --install`
    (the CLT prompt also provides the compiler some wheels need), polling for
    git to appear.
  - Linux: apt/dnf/pacman via sudo when available.
Falls back to the manual instructions only if auto-provision fails.

* feat(desktop): in-app GUI+backend self-update on macOS/Linux

On Windows the staged Hermes-Setup binary drives updates (quit → hermes
update → hermes desktop --build-only → relaunch). The mac drag-install has no
such binary, so "Update now" previously just printed `hermes update`.

Since there's no venv-shim file lock on POSIX, the desktop can drive the whole
update itself. applyUpdates now, when no staged updater exists on mac/linux:
  1. runs `hermes update --yes [--branch <current>]` (backend git pull + deps),
  2. runs `hermes desktop --build-only` (OS-aware GUI rebuild) with the
     Hermes-managed Node + venv on PATH,
  3. spawns a detached swapper that waits for this process to exit, dittos the
     freshly built Hermes.app over the running bundle, clears quarantine, and
     relaunches.
Degrades to "backend updated — restart to load the new GUI" if the rebuild
fails or there's no .app bundle to swap (dev run, Linux AppImage).

* chore: uptick

apps/bootstrap-installer/src-tauri/src/paths.rs

@@ -2,8 +2,15 @@
 //!
 //! Mirrors `hermes_constants.get_hermes_home()` from the Python CLI:
 //!   Windows: %LOCALAPPDATA%\hermes
-//!   macOS:   ~/Library/Application Support/hermes
-//!   Linux:   ~/.hermes  (XDG override via $HERMES_HOME)
+//!   macOS:   ~/.hermes
+//!   Linux:   ~/.hermes  (override via $HERMES_HOME)
+//!
+//! NOTE (macOS): Python's get_hermes_home(), scripts/install.sh, and the
+//! Electron desktop's resolveHermesHome() ALL use ~/.hermes on macOS — there
+//! is no ~/Library/Application Support branch anywhere else. An earlier
+//! version of this file used Application Support, which drifted from every
+//! other component: the installer wrote the install to one dir and the
+//! desktop looked for it in another, so first launch never found the backend.
 //!
 //! IMPORTANT: this must match exactly. Drift here means install.ps1
 //! writes to one place and the installer reads from another, breaking
@@ -28,15 +35,8 @@ pub fn hermes_home() -> PathBuf {
         }
     }
 
-    #[cfg(target_os = "macos")]
-    {
-        // ~/Library/Application Support/hermes
-        if let Some(home) = dirs::home_dir() {
-            return home.join("Library/Application Support/hermes");
-        }
-    }
-
-    // Linux + fallback: ~/.hermes
+    // macOS + Linux + fallback: ~/.hermes (matches Python get_hermes_home(),
+    // install.sh, and the Electron desktop's resolveHermesHome()).
     if let Some(home) = dirs::home_dir() {
         return home.join(".hermes");
     }