feat: real designs for Keystone hardware wallet signup and signing

[n13]

Replaces the dev-design Keystone screens with the real designs from the Quantus Redesign Figma.

Screenshots: See below (simulator)

Designs:

• Add keystone: https://www.figma.com/design/D8yu03ee9MXPboWIqqCrfX/Quantus-Redesign?node-id=4184-14591&m=dev
• Send signing flow: https://www.figma.com/design/D8yu03ee9MXPboWIqqCrfX/Quantus-Redesign?node-id=4184-15239&m=dev

Add Keystone flow (2 screens)

• AddHardwareAccountScreen rewritten as the intro screen: hero product image, headline, "Connect Hardware Wallet" button, and a "Don't have one? Get a Keystone ↗" link. Constructor unchanged, so existing callers (add-account menu, create-account) are untouched.
• New ConnectKeystoneScreen: firmware reminder + on-device steps with the Figma icons, then "Ready to Scan" opens the QR scanner. The name/address form is gone — the account is created directly after scanning (name defaults to "Keystone Wallet"). Debug test-address path kept in kDebugMode.

Send signing flow — now a 3-step journey

Most of the send flow is unchanged; the branch where the user signs with Keystone is redesigned:

• Step 1/3 KeystoneSignScreen: step indicator, "Scan with your Keystone", bordered QR card, "YOU ARE SIGNING" details (amount/recipient + checksum), "Continue to Sign" / "Cancel Transaction".
• Step 2/3 new KeystoneVerifyScreen: "Check your Keystone Screen" — dark summary card with amount and recipient, red mismatch warning, "Continue to Sign" / "It doesn't match".
• Step 3/3 AnimatedQrScanner restyled: app bar, step header, rounded camera view with corner brackets, "RECEIVING SIGNATURE" progress bar.
• New KeystoneRejectScreen ("It doesn't match"): reject-on-device instructions, "I rejected it on Keystone", "Contact Support" mailto.

Multisig hardware signing gets the same new flow automatically since it shares these screens.

Supporting changes

• New textSubtle color in AppColorsV2, shared KeystoneStepLabel / KeystoneWarningCard widgets, UnderlinedTextLink component
• Figma assets (hero image, logo, step icons) added to assets/v2/
• New/updated l10n strings; stale Indonesian overrides for changed/removed strings dropped (fall back to English)
• Removed unused KeystoneSigningSession.title (app bar is "Sign with Keystone" per design)

Deliberate deviations from Figma

• Step 2 "≈ $10" fiat row omitted — no fiat data in the signing session
• Scanner torch button dropped to match the design

Testing

• melos run format clean; dart analyze --fatal-infos clean on lib/ and test/ (melos' own analyze run kept crashing its analysis server on mobile-app — env flake, not a code issue)
• Not yet tested on device — set AppConstants.debugHardwareWallet to force the Keystone path for testing without hardware

[n13]