Add --branch support to install script with validation

  The install script hardcoded origin/main, so beta testers told to
  install from a dev branch silently got the stable release instead.
  Add a --branch CLI option and interactive prompt (defaults to main).
  Invalid branch names are validated via git ls-remote before any work
  is done, showing available branches on failure.

CHANGELOG.md

@@ -8,7 +8,7 @@ All notable changes to Bambuddy will be documented in this file.
 - **AMS Info Card & Custom Labels** ([#570](https://github.com/maziggy/bambuddy/pull/570)) — Hovering an AMS label (e.g. "AMS-A") on the Printers page now shows a popover with serial number, firmware version, and an editable friendly name. Custom labels are stored by AMS serial number so they persist when the unit is moved to a different printer. Slot numbers are now displayed inside each filament color circle with auto-inverted contrast for readability. Labels also appear in the Inventory page's location column. Contributed by @cadtoolbox.
 
 ### Improved
-- **Install Script: Branch Selection** — The native install script (`install.sh`) now supports a `--branch` option and an interactive branch prompt (defaults to `main`). Previously the script hardcoded `origin/main`, so beta testers told to install from a beta branch would silently get the stable release instead. Fresh installs use `git clone --branch`, existing installs checkout and reset to the selected branch. The install summary highlights non-main branches in yellow with a "(beta)" label.
+- **Install Script: Branch Selection** — The native install script (`install.sh`) now supports a `--branch` option and an interactive branch prompt (defaults to `main`). Previously the script hardcoded `origin/main`, so beta testers told to install from a beta branch would silently get the stable release instead. Fresh installs use `git clone --branch`, existing installs checkout and reset to the selected branch. The install summary highlights non-main branches in yellow with a "(beta)" label. Invalid branch names are caught early with an error message listing available branches.
 - **Print Queue Scheduler Diagnostics** ([#616](https://github.com/maziggy/bambuddy/issues/616)) — Added diagnostic logging to the print queue scheduler to help diagnose why queued prints aren't starting. After each queue check, the scheduler now logs a skip summary (how many items were skipped due to manual_start, scheduled_time, etc.) and for each busy printer, logs the exact state preventing it from being considered idle (connected status, printer state, plate_cleared flag). Previously the scheduler only logged "found N pending items" with no visibility into why items were skipped.
 
 ### Fixed

install/install.sh

@@ -335,6 +335,14 @@ create_user() {
 download_bambuddy() {
     log_info "Downloading BamBuddy..."
 
+    # Validate branch exists on remote before proceeding
+    if ! git ls-remote --exit-code --heads https://github.com/maziggy/bambuddy.git "$BRANCH" &>/dev/null; then
+        log_error "Branch '$BRANCH' not found in the BamBuddy repository."
+        log_info "Available branches:"
+        git ls-remote --heads https://github.com/maziggy/bambuddy.git | sed 's|.*refs/heads/|  - |'
+        exit 1
+    fi
+
     if [[ -d "$INSTALL_PATH/.git" ]]; then
         log_info "Existing installation found, updating..."
         # Add safe.directory to avoid "dubious ownership" error when running as root