feat(demo): funded-wallet restore + build-arg fixes

Adds DEMO_WALLET_SEED/DEMO_WALLET_HEIGHT support so a demo build restores a
fixed (throwaway, funded) Wownero wallet instead of creating a fresh one, via
WalletRestoreViewModel.create. Balance lives on-chain against the seed, so it
survives any emulator wipe and is reproducible.

Three real bugs fixed on the way:
- build-apk.sh built the --dart-define args as a word-split string; the 16-word
  seed's internal spaces split into separate argv tokens and flutter treated a
  seed word as a build target ("Target file '<word>' not found"). Now a bash
  array keeps each --dart-define one token.
- WalletRestoreViewModel.getCredentials reads name/passphrase from the options
  MAP (not the vm field) and casts name as non-null String, so the restore
  options must include 'name' and 'passphrase'.
- forward DEMO_WALLET_SEED/HEIGHT into the build container; source demo.env.

Verified end to end: demo build restores the wallet, its address matches the
funded one exactly (QR-decoded), it syncs at height 861800, and the dashboard
shows the real 13.37 WOW balance + tx history. Send shows Max 13.37.

The seed lives only in gitignored tools/demo/demo.env (never committed).
docs/internal-notes.md (gitignored) documents the FailedDecryption gotcha.

.gitignore

@@ -244,3 +244,6 @@ scripts/windows/setup-windows-runner.ps1
 # committed template is tools/demo/demo.env.example.
 tools/demo/demo.env
 __pycache__/
+
+# Internal engineering notes (not for the public repo)
+docs/internal-notes.md