Add progress feedback for file operations. Depends on #90.

[Jens-Olaf-Mueller]

This adds responsive progress feedback for loading and renaming large file lists.

Changes:

• add a progress bar next to the file toolbar
• process loading and renaming incrementally via GLib.idle_add()
• keep the GTK UI responsive during long operations
• show progress as current / total
• temporarily disable file controls while processing
• prevent parallel load/rename jobs from drag-and-drop, keyboard shortcuts or toolbar actions
• restore UI state correctly after completion or errors
• update bulky.pot with the new translatable progress strings

Tested successfully with:

• 2,033 files
• 4,307 files
• repeated loading
• repeated renaming
• toolbar locking during active operations