Refactor and unify the send & multisig-propose flow

[n13]

Summary

This branch started as a one-line fix for the scan→continue button bug, but the scope grew into a full refactor of the send experience. The regular send and multisig propose flows were near 1:1 duplicated screens; they are now unified behind a single set of screens driven by a SendStrategy, and several related fixes/features came along with it.

Net ~−800 lines (four duplicated multisig screens deleted), no new localization keys, and the regular send flow is visually unchanged from the original Figma design.

1. Unified send & multisig-propose flows (Strategy pattern)

The two flows (recipient → amount → review → done) were copy-pasted. They now share one implementation, with a SendStrategy encapsulating everything that differs between them:

• send_strategy.dart — SendStrategy + value types: SendStrings (per-flow labels), SendFee (RegularFee / ProposeFee), SendOutcome (Submitted / NeedsHardwareSignature / Failed), SendTerminalContent.
• regular_send_strategy.dart — single-signer transfer from the active account (local signing, or Keystone hand-off).
• multisig_propose_strategy.dart — multisig proposal (member affordability gating, propose-fee breakdown, proposal submission + provider invalidation).
• Shared screens now take a strategy: select_recipient_screen.dart, input_amount_screen.dart, review_send_screen.dart, and a unified send_terminal_screen.dart (renamed from tx_submitted_screen.dart).
• Deleted the duplicated multisig screens: propose_recipient_screen.dart, propose_amount_screen.dart, propose_review_screen.dart, propose_done_screen.dart.
• Entry points updated: home_screen.dart, shared_address_action_sheet.dart.

Keystone signing seam: hardware signing is modeled as an orthogonal step via SendOutcome.NeedsHardwareSignature, threaded through keystone_sign_screen.dart / keystone_scan_signature_screen.dart. This leaves room for multisig + Keystone later without adding another branch.

2. "View in Explorer" on the terminal screen

• The success screen now shows a "View in Explorer ↗" link built from the extrinsic hash returned by submission.
• transaction_submission_service.dart now returns the extrinsic hash (it was already computed, just discarded); added an explorerImmediateTransactionUrl() helper.
• Extracted a shared ExplorerLink widget and reused it across the terminal, the POS receipt (pos_qr_screen.dart), and the transaction/proposal detail sheets — removing three near-duplicate copies.

3. Self-send handling on the recipient screen

Entering or scanning your own address previously left the button silently disabled on "Enter address". Now it:

• labels the (still-disabled) button "Can't Self Transfer", and
• shows a warning toaster once on entry.

Works for both flows (strategy.sourceAccountId resolves to the active account for sends, the multisig address for proposals).

4. Debug QR scanner button

• Added a kDebugMode-only button to the shared QrScannerPage that returns a test address, so the send / swap / add-account flows can be exercised in the simulator where the camera is unavailable.
• Shared AppConstants.debugTestAddress, reused by the existing hardware-account debug fill.

5. Original fix (scan → continue button)

Scan and paste took different code paths; scan assigned the controller text inside setState, so the validation listener never enabled the button. Unified scan/paste/recent through a single _setRecipient helper (the path paste already used). This screen was subsequently folded into the refactor above.

Test plan

• Regular send — recipient (scan / paste / recent / type) → amount → review → submit → terminal shows "sent" with a working explorer link
• Regular send via Keystone — review hands off to the QR sign flow → broadcasts → terminal + explorer link
• Multisig propose — recipient → amount → review → propose → terminal; proposal shows as pending
• Self-send — entering/scanning own address shows "Can't Self Transfer" + toaster, button stays disabled (both flows)
• Explorer link — terminal, POS receipt, and tx/proposal detail sheets all open the correct explorer URL
• Pay mode (payment-URL QR) — recipient + amount prefilled
• Regular send flow visually matches the original Figma design