[turinglambdaai]

Cross-platform task interaction parity

GTK task lists already have native multi-selection and batch complete/move (#33/#43), but lack bulk due-date scheduling. This PR adds the same first-stage Schedule Selected action as Windows #45, including date/time consistency.

Changes

• Add localized batch-schedule button to the GTK multi-selection action bar.
• Provide Today, Tomorrow and Clear options via a native popover.
• Freeze task IDs when the popover opens, and re-resolve each task before updating: don't mutate unrelated tasks if the search or list changes while the menu is visible.
• Clear resets due_date and due_time together, preserving title, notes, owning list and completion.
• Reuse typed update_task API and the application's error handling.
• Update the UX benchmark to distinguish shipped controls from remaining parity gaps.

Dependencies / coordination

Stacked on #43 → #33. The zh/en string bulkSchedule is also added by Windows #45. Resolve the shared i18n addition once during integration; verify bytes on every platform. Other Linux PRs #34, #38 and #44 touch the same source file and need a combined build.

QA before merge

• Linux GTK host CI and packaged Product Acceptance pass
• Select two pending tasks → Tomorrow: both scheduled for next day, no change in list ownership
• Assign due times then Clear: both date/time fields null after restart
• Do the same from a global search containing Work and Life tasks
• Change selection/search while the popover is open: only originally selected IDs are eligible
• Chinese and English UI, keyboard focus, light/dark

Still outstanding: safe bulk delete/undo and Windows inline detail editor. Do not release based on this PR alone.