WebGL: Add offline support

[melekr]

Context

WebGL users can lose unsent Backtrace reports when:

• BacktraceDatabase writes aren’t flushed from IDBFS to IndexedDB before tab close/background.

and/or

• BacktraceDatabase is disabled/unavailable, causing network/server send failures to be dropped.

Solution

Persist offline reports reliably (IDBFS flush hooks + PlayerPrefs fallback queue)

This PR adds two WebGL-specific reliability layers consistent with existing Backtrace database settings:

1. IDBFS flush support

• Adds BacktraceSync.jslib exposing FS.syncfs
• Adds BacktraceWebGLSync.cs wrapper
• Installs page lifecycle hooks (pagehide, beforeunload, visibilitychange, freeze) and calls FS.syncfs at key SDK lifecycle points to reduce data loss.

2. Fallback offline queue when BacktraceDatabase is unavailable:

• Adds WebGLOfflineDatabase.cs PlayerPrefs queue.
• Queue bounds derive from BacktraceConfiguration database settings (MaxRecordCount, MaxDatabaseSize, RetryLimit, RetryOrder, RetryInterval, AutoSendMode)
• Enforces WebGL safe hard caps to avoid excessive PlayerPrefs/browser storage use
• Replays queued reports on an interval when AutoSendMode is enabled.

Notes / Limitations

• FS.syncfs is best-effort and async: abrupt tab termination can still lose data.
• PlayerPrefs queue is intentionally bounded: oversized reports are not persisted.

Sample Reports

ref: BT-6231