feat: platform-wallet backend rewrite (spec + implementation)

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
+
+## [Unreleased]
+
+### Changed
+
+- **Sign-time wallet unlock**: the passphrase is now requested just-in-time, the
+  moment an operation actually needs your secret (sending funds, registering an
+  identity, signing). The prompt offers an optional "Keep this wallet unlocked
+  until I close the app" checkbox so a busy session asks only once. This replaces
+  the old upfront unlock gate that held the wallet open for the whole session —
+  the HD seed is no longer kept in memory between operations; it is decrypted on
+  demand and wiped as soon as the operation finishes.
+
+- **Wallet storage backend**: HD wallet seeds and single-key private keys are now
+  stored in an upstream `platform-wallet-storage` encrypted vault (`secrets.pwsvault`)
+  rather than in the legacy `data.db` SQLite database. Wallet metadata (alias, main flag,
+  Core wallet name) moves to a new `det-app.sqlite` key-value sidecar. The legacy
+  `data.db` file is left intact for safety; it is no longer read at runtime.
+
+- **Cold-start migration**: on the first launch after upgrading, DET automatically
+  migrates wallet seeds, metadata, and imported single-key data from `data.db` into
+  the new storage layout. A progress banner is shown during migration (typically
+  under one second on local storage). The migration is idempotent — subsequent
+  launches skip it via a completion sentinel in `det-app.sqlite`.
+
+### Removed
+
+- Proof log screen (internal developer tool, not part of the public feature set).
+- QR-code wallet import flow for identity funding and top-up screens.
+
+### Fixed
+
+- `WalletBackend` is now initialised eagerly at `AppState` start, eliminating a
+  retry-loop spam on the SDK connection during cold boot.
+- Wallet store is rehydrated on cold start, resolving a regression where wallets
+  were not visible after the storage migration.