chore: rename the product to mintlayer nexus

[erubboli]

What

Product rename: Mintlayer GUI-X → Mintlayer Nexus (decided in the naming vote; shipping now that the fix wave has settled), plus the version bump to 1.0.0-rc2 so the rc2 artifacts are the first Nexus-branded release.

All 15 user-visible sites, mechanical string change only:

• Tab titles + sidebar + mobile menu + footer (Layout.astro:80/96/312/335 — footer now renders Mintlayer Nexus v1.0.0-rc2 from package.json)
• Login page title + header (login.astro:155/165)
• TOTP enrollment labels (login.astro:218 QR, settings.astro:62, reset-2fa.ts:32, init.sh:344)
• Telegram bot greeting + test messages (telegram-commands.ts:87, settings/telegram.ts:35)
• WebAuthn rpName (passkey/register-options.ts:41)
• README title + init.sh setup banner

Compatibility (no migrations, nothing breaks)

• Passkeys: rpName is display-only; rpId (the domain) is unchanged — all existing passkeys keep working.
• TOTP: the otpauth label/issuer is display-only — existing authenticator entries keep generating valid codes; new enrollments show "Mintlayer Nexus".
• No persisted data carries the product name — session cookie, prefs keys and localStorage keys are name-neutral.
• Out of scope: repo name, package name, image paths (watchtower/installs unaffected). The Telegram bot display name in BotFather is a manual follow-up.

Verification

• grep -ri 'gui-x' across app/, init.sh, README → 0 hits
• tsc clean · 74 files / 879 tests green · astro build OK · bash -n init.sh
• Diff reviewed in a recursive findings loop to zero findings