docs: rework the README after review
Corrects every statement the review found false against the code, among them which methods the background relays and for whom, where Connected Sites live, which fields the stored-data check and the defaults cover, what the balance lists round, and which address rows are plain text. Puts the test-file citations under Tests, defines each technical word on first use, turns every comma-chained list into bullets, and adds the coverage the review asked for: valid amounts, the Firefox minimum version, the RPC privacy note, the schema-version rule, the browser-suite rules. Part of #672. Model: fable-5-1
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