Fix crashes, logic errors, and lifecycle bugs

[sreckoskocilic]

Summary

• _on_file_removed crash — null-guard sourceModel() before accessing location; iterative parent traversal replaces recursive call that could stack-overflow
• _MAX_VISITED cap on _already_visited set (512 entries) to prevent unbounded memory growth
• SortedFileSystemModel.shutdown() — proper cleanup: disconnect signals, remove callbacks, shut down source model
• SourceFileLoader → importlib.util — deprecated loader replaced with spec_from_file_location for correct subpackage resolution
• ListenerWrapper thread pool — unbounded Thread(daemon=True) replaced with bounded ThreadPoolExecutor(max_workers=1) with shutdown_pool() for clean exit
• ReportExceptions as context manager — stores .exception attribute; command_registry uses it instead of bare except swallowing all errors
• _distribute_evenly / _distribute_exponentially — remainder pixels now distributed so sum(result) == width
• _resize_column overflow — shrink path clamped to available_width so columns can't exceed viewport
• restoreState / saveState — single-byte length encoding replaced with struct.pack('<I', ...) to support state > 255 bytes; restoreState wrapped in try/except for corrupted state
• overlay.move() float → int — explicit int() casts prevent Qt deprecation warnings
• _from_human_readable — relative vs absolute path detection fixed; absolute paths no longer incorrectly joined with src_dir
• GoBack / GoForward — .get() instead of [] to avoid KeyError when history listener is missing
• _Delete.show_alert — instance method call instead of module-level, respects subclass overrides
• OpenFile null guard — early return when no file is under cursor
• zip.py plugin install — validates exactly one top-level directory in zip before extracting
• drop handler — catches ValueError from malformed QUrls instead of crashing
• uniform_row_heights — default=-1 prevents ValueError on max() of empty sequence
• key_bindings — elif fixes unreachable branch for empty keys validation
• quicksearch — guard against row_height <= 0 before computing max height
• onboarding — unclosed </ul> tag now properly closed
• application_context cleanup — model shutdown on quit, metrics shutdown, LD_LIBRARY_PATH docstring condensed with updated URL
• session.py — is_first_run property used instead of re-checking settings; save failure logged instead of silently swallowed
• goto.py — duplicate inode check moved before to_visit.append to avoid redundant work; stale TODO removed
• email URL encoding — quote(email, safe='') in splash screen login link
• ProgressDialog thread safety — Lock guards _text, _progress, _size; null-check on cancel button
• FilterBar — null-guard sourceModel() before calling update()
• OS X → macOS — terminology updated in comments
• assert → raise — 5 bare asserts replaced with proper exceptions
• quicksearch_matchers — contains_chars / contains_substring fixed: case-insensitive matching, proper index tracking
• Unused files variable removed from clipboard copy command
• Unused basename import removed from core/commands/__init__.py

Test plan

• Open and navigate directories — verify no crashes on file removal or parent traversal
• Resize columns in file list — verify no overflow beyond viewport
• Install plugin from zip — verify validation of zip contents
• Drag and drop files with malformed URLs — verify graceful handling
• Test quicksearch with various inputs — verify case-insensitive matching works
• Verify window state save/restore across restarts