Harden sync, clipboard, and large-tab behavior

[FreshSoftware4]

Summary

This draft consolidates the CopyQ bugfix pass described in the supplied sync-bug exports and rebases it onto the current upstream master with normal ancestry.

It:

• makes ItemSync ownership, lazy materialization, atomic writes, URI imports, and filesystem reconciliation failure-aware
• prevents destructive item commands from deleting synchronized items whose backing data cannot be read
• hardens clipboard reads and retries, including Windows native format checks and WebP/GIF handling
• improves large-tab responsiveness with batched placeholder/layout work and sparse item-widget tracking
• fixes move-to-top-after-paste behavior, translation-independent shortcuts, window-state restoration, tag sizing, selected-tab v3 imports, and script-dialog sizing
• adds focused regression coverage for the affected sync, clipboard, image, import/export, shortcut, command, item, and script paths

Why

The original failure mode combined network-backed synchronized items, transient or permanent read failures, and code paths that treated missing data as an empty item. That could allow later command/removal paths to discard an item even though its synchronized payload had never been materialized successfully. Related clipboard and large-tab behavior amplified the reliability and responsiveness problems documented in the reproduction exports.

During review, two additional holes were found and fixed:

1. command actions using remove still ignored typed selection-copy failures, including remove-only commands
2. trailing-row deletion could leave stale entries in the sparse item-widget index map

Validation

• git diff --check passes
• the supplied Git bundle verifies as complete
• CMake was run with the repository required Debug/Ninja/test configuration under an initialized MSVC environment
• configuration reached dependency discovery but cannot complete on this machine because the Qt 6 development package (Qt6WidgetsConfig.cmake, Qt >= 6.2) is not installed
• runtime tests were therefore not run locally; the focused tests in this branch and Windows/SMB reproduction still need CI or a Qt-enabled environment

This is intentionally a draft while CI and maintainer review establish whether the broad bugfix pass should remain together or be split into smaller PRs.