fix: reduce CPU usage on Manage page (#1998)

[gauravsaini]

Summary

• Fixes high CPU usage (10%+) on the Manage screen when many snaps are installed
• Fixes High CPU usage on "Manage" screen #1998

Root Cause

The ManageAppActions widget called ref.watch(snapModelProvider(snap.name)) for every snap tile, creating a full SnapModel for each installed snap. With 100+ snaps, this caused significant overhead as each SnapModel independently fetches data from snapd.

Fix

Refactored _buildSnapActions to avoid creating per-snap SnapModel instances on each tile:

1. Display decisions use Snap data directly - refreshInhibit, apps.isNotEmpty, updateVersion are available from ManageSnapData.snap without needing a full SnapModel

2. Active changes tracked at manage page level - Uses currentlyInstallingModelProvider instead of per-snap SnapModel.activeChangeId

3. Lazy SnapModel creation - Only creates/reads snapModelProvider when an action button (update, open, remove) is actually pressed

4. Simplified launchability check - Uses snap.apps.isNotEmpty instead of LaunchProvider

Testing

Verify that:

• Manage page loads quickly with 100+ installed snaps
• Update, Open, and Remove buttons still work correctly
• Active change status still shows when a snap operation is in progress