Fix SingleApplication IPC reliability and add font style selection

[blreay]

1. Fix unreliable SingleApplication IPC message delivery

The default sendMessage() timeout of 100ms is insufficient when the primary instance's event loop is temporarily blocked by session auto-save (every 60s), file-change detection, or modal dialogs. This causes the secondary instance to activate the primary's window (via instanceStarted signal) but fail to deliver the file path (via sendMessage), resulting in "window activates but file doesn't open" behavior.

• Increase sendMessage() timeout from 100ms to 1000ms per attempt
• Add retry logic (3 attempts) to handle transient event-loop blocking
• Show QMessageBox warning on total failure instead of silent exit
• Change sendInfoToPrimaryInstance() return type to bool

Ref: upstream SingleApplication issue, NotepadNext #541

2. Add font style selection to support Light/Thin font variants

Qt 6 merges font family variants (Regular, Light, Bold, etc.) under a single family name in QFontComboBox, making variants like 'Sarasa Fixed SC Nerd Font Light' invisible in the font picker.

• Replace QFontComboBox with QComboBox + style QComboBox in Preferences
• Populate families via QFontDatabase::families(), styles via QFontDatabase::styles(family) with cascading updates
• Add FontStyle setting to ApplicationSettings (persisted as style name)
• Map selected style to Scintilla weight/italic via styleSetWeight() and styleSetItalic() in EditorManager (initial setup + live updates)
• Use this->settings in lambdas for MSVC -permissive- compatibility

3. Add documentation for both changes in doc/