Make notification detection more robust

[GyaneshSamanta]

src/webview-preload.js watches the entire DOM with a broad MutationObserver and uses fragile heuristics like class-name matching and short text checks.

This can miss real NotebookLM notifications or fire on unrelated UI changes.

A stronger approach would:

• Narrow the selectors.
• Deduplicate repeated events.
• Reduce false positives.
• Make the detection logic easier to maintain as NotebookLM changes.