Add minimal read-only timeline widget with playback playhead

[DhyaniKavya]

issue: #5116

Problem:
Music Blocks currently does not provide any way for users to keep track of earlier versions of their projects. If a user experiments or makes a mistake, there is no way to review past states or understand how the project evolved over time.
This makes experimentation risky, especially for learners and educators.

Solution:
This PR introduces a minimal, read-only project snapshot history feature.
Users can manually save snapshots of their project and later view a list of those snapshots without affecting the current project state. The feature is intentionally limited in scope to avoid any impact on existing behavior.

What this adds?
-A small storage layer to save project snapshots locally using IndexedDB
-A read-only snapshot history widget that lists saved snapshots
-Each snapshot includes:

->Timestamp
->Optional user description
->Associated project ID
->Snapshots persist across page reloads and work offline

What this does not do?
-No restore or rollback functionality
-No deletion or cleanup logic
-No changes to playback, blocks, or project behavior
-No changes to existing save/load logic

Implementation Details:-
New files:

->SnapshotStorage.js:
->Handles saving and reading snapshots using the same IndexedDB setup already used in the project
->Exposes simple methods to save and fetch snapshots
->snapshothistory.js:
->A new widget following the same pattern as existing widgets (e.g. Status Matrix)
->Displays a read-only list of snapshots with basic metadata
->Viewing a snapshot only shows details, it does not modify the project

Modified files:-
-activity.js
-Initializes snapshot storage
-Adds helper methods to save snapshots and open the snapshot history widget
-index.html
-Loads the new snapshot-related scripts
-All changes are localized and follow existing project patterns.

Verification:
-Saved snapshots appear correctly in the snapshot history widget
-Viewing a snapshot does not alter the current project
-Playback and block behavior remain unchanged
-Snapshots persist after page reloads
-Feature works offline
-No console errors observed

AI Disclosure:-
-AI tools were used only for discussion and planning.
All implementation decisions, scoping, and verification were done manually.

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.