Adopt new style guideline for capitalization: "Title Case" titles vs "Sentence case" buttons

[kdmukai]

TopNav titles should be "Title Case" where every major word (and, for now, every minor word, too) should be capitalized.

Only aware of one English screen that needs to be fixed: https://github.com/SeedSigner/seedsigner-screenshots/blob/dev/en/seed_views/SeedAddPassphraseExitDialogView.png

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But buttons should be "Sentence case": only the the first word is capitalized (though proper nouns like "SeedQR" should always retain their capitalization).

There are many buttons that need capitalization fixes.

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Some proper noun judgment calls:

• "Xpub" always capitalized? I vote no.
• "BIP-39 Passphrase"? Probably yes because it's such an important topic to get right.
• "Native Segwit" / "Nested Segwit" / "Taproot"? Yes? They are crucial concepts, even if they're not always capitalized elsewhere in the bitcoin world.
• "Custom derivation"? No need to always capitalize.

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related: SeedSigner/seedsigner-translations#8

[jdlcdl]

Yes, YES, 100%!

Guidelines that are strict and implemented in English will also be picked up by detail-oriented translators where applicable. While I don't understand Spanish, I'm picking-up that this is getting discussed in that tg group too. It's helpful as I'll make similar suggestions in French.