Add toggle to enable/disable automatic update checking

[leofrederiche]

Summary

• Add a new GSettings key auto-check-updates (default: true) to control whether the app automatically checks for extension updates on startup
• Add an Adw.SwitchRow toggle in the Installed page UI, with an icon and translatable strings, bound bidirectionally to the setting
• Wire the auto-check-updates property into ExmInstalledPage so that exm_manager_check_for_updates() is only called when the setting is enabled

Motivation

Give users greater control over when updates should occur. Users who have found a stable environment can disable automatic update checking to avoid unexpected changes that could disrupt their workflow.

Changes

File	Description
data/com.mattjakeman.ExtensionManager.gschema.xml.in	New boolean key auto-check-updates (default true)
src/exm-installed-page.blp	New Adw.SwitchRow widget with icon and i18n labels
src/exm-installed-page.c	New auto-check-updates property, GSettings/GObject bindings, conditional update check
po/extension-manager.pot	Updated translation template with new strings

Test plan

• Launch the app with the default setting — updates should be checked automatically on startup (existing behavior preserved)
• Toggle "Auto Check for Updates" off, restart the app — updates should not be checked on startup
• Toggle it back on, restart — updates should be checked again
• Verify the setting persists across sessions via gsettings get com.mattjakeman.ExtensionManager auto-check-updates
• Verify the toggle appears correctly in the Installed page with the software-update-available-symbolic icon

Note

This is my first contribution to an open-source project. Claude was used as an aid during the development of this feature.