Add Shift+Enter shortcut for backward search

[saberoueslati]

Description

Fixes #1082

The Find/Replace dialog currently requires manually enabling Backward direction to search for the previous match. This PR adds browser-style keyboard navigation: Enter keeps its existing behavior, while Shift+Enter searches backward.

Shift+Enter searches backward regardless of the Backward direction checkbox state. In regular expression mode, where backward searching is unsupported, it falls back to a forward search.

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Changes

src/dialogs/FindReplaceDialog.cpp

• Added dialog-wide shortcuts for both Shift+Return and Shift+Enter.
• Used Qt::WidgetWithChildrenShortcut so the shortcut works even when a button or another dialog control has focus.
• Limited the shortcut to the Find and Replace tabs by checking whether the Find button is visible, keeping the Mark tab behavior unchanged.
• Extracted the existing search logic into performFind(bool backwards).
• Preserved the existing Find button behavior, including support for the Backward direction checkbox.

src/dialogs/FindReplaceDialog.h

• Added the private performFind(bool backwards) helper declaration.

Manual test after the fix below :

Screencast.from.2026-07-13.00-52-07.webm