Fix bugs found by code quality scans

[sreckoskocilic]

I forked this repo in order to build myself a new fman version. Prior to making any build I ran the code-quality-scans performed by claude AI. This PR contains some fixes for the most critical things found in the scan.

• The comma-separated return creates a 7-element tuple instead of comparing two tuples, every equality check returns True
• master -> main branch fix
• prepare_trash now calls self.move_to_trash instead of self.delete, so plugin subclasses won't permanently delete files when the user expects trashing
• NotImplementedError - unrecognized platforms fail fast with a clear message instead of a cryptic NameError
• removed shadowing basename import
• get_column_widths uses range so plugin-added columns get their widths saved/restored
• 2 failing tests fix
• all pending plugin errors are now shown at startup instead of just the first one
• command_registry.py — _set_context now uses try/finally so cm.exit always runs, even on exception.
• util/qt/init.py — Added missing c_void_p import from ctypes, fixing a macOS runtime crash.
• table.py — Fixed off-by-one: bounds check now rejects len + 1 correctly.
• widgets.py — Added null guard on _main_window before accessing it in state change handler.
• util/path.py — normalize now loops until all .. segments are resolved, so a/b/c/../../d correctly becomes a/d.
• session.py — Removed dead _get_startup_message method (duplicated by _show_startup_messages)