[nai62]

• Add an option under General to move the window by dragging an image or its surrounding margin. It defaults to off and is saved with the other viewer settings.
• Use Qt's system move operation after the normal drag threshold. Preserve side-click page turns, image drag-scrolling, loupe operations, and assigned mouse actions; keep window movement disabled in fullscreen. Assigned clicks use the mouse event's modifiers so their lookup agrees with the drag decision even when the current keyboard state differs.
• Add viewport regressions for an empty viewer, opening an image, margins, cursor scrolling, transformed layouts, option changes, cancelled gestures, unsupported moves, and modifier-sensitive click assignments.
• Add the Japanese label and tooltip, register the two strings as unfinished in the other six catalogs, and document Windows verification.
• Checked the Windows Debug build and 384 cases across windowstartup (138), viewernavigation (241), and languageswitch (5), with no failures or skips. Existing C4651/C4996 compiler warnings and Qt fixture/resource, geometry, and temporary-directory cleanup warnings remain. Interactive Windows checks and macOS/Linux behavior are still unverified.