Rebuild as single-file offline BIP84 wallet

Replace the SvelteKit/daisyUI prototype with a vanilla JS + hand-rolled CSS
app that Bun bundles (code + scure/noble crypto + QR + CSS) into one
self-contained dist/index.html runnable offline from file://.

Wallet core (src/wallet.js):
- BIP84 native-segwit HD wallet from BIP39 seed (+optional passphrase)
- gap-limit scan of receive/change chains, aggregate balance, tx history
- fresh receive/change addresses; signs with witnessUtxo only (enables
  offline signing with no prev-tx fetch)
- coin selection + fee estimation via btc-signer selectUTXO; send-max;
  manual coin control
- offline JSON snapshot export/import (keyless) + signed-tx hex/QR export
- realtime mempool.space WebSocket: track-addresses -> debounced auto
  refresh of balances/history (no polling); live status badge

UI (src/app.js): clean light paper-wallet; create (with optional, skippable
backup verification) / import flow; receive (QR) / send / history / coins /
tools tabs; clickable logo returns home.

API (src/api.js): mempool.space wrapper with retry/backoff for scan-burst
rate limiting; single network choke-point for offline enforcement.

Verified: BIP84 spec vectors, build/sign/parse for auto/max/manual flows,
snapshot roundtrip, live scan + WebSocket, headless-Chrome UI smoke tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

1 files changed, 61 insertions, 23 deletions

diff --git a/README.md b/README.md
index b5b2950..3821f32 100644
--- a/README.md
+++ b/README.md
@@ -1,38 +1,76 @@
-# sv
+# Bitcoin Wallet
 
-Everything you need to build a Svelte project, powered by [`sv`](https://github.com/sveltejs/cli).
+A self-contained, **single-file** Bitcoin wallet that runs entirely in the
+browser. Think bitaddress.org, but a modern BIP84 HD wallet built from a seed
+phrase, able to scan history, fetch UTXOs, and spend — online or fully offline.
 
-## Creating a project
+- **BIP84 / native SegWit (p2wpkh)** HD wallet from a 12/24-word BIP39 seed
+  (optional passphrase).
+- **HD scanning** with the standard gap limit across the receive (`…/0/i`) and
+  change (`…/1/i`) chains — real aggregate balance, fresh addresses, and
+  transaction history via [mempool.space](https://mempool.space).
+- **Spending** with proper coin selection + fee estimation
+  (`@scure/btc-signer`'s `selectUTXO`), a fee-rate picker, send-max, and
+  **manual coin control**.
+- **Fresh addresses** for receiving and for change — no address reuse.
+- **Offline mode**: import UTXOs from a JSON snapshot (fetched on an online
+  device), sign locally, and export the signed transaction (hex / file / QR) to
+  broadcast elsewhere. No network calls are made.
+- **Runs offline from `file://`** — keys never leave the page; nothing is sent
+  anywhere except, when online, read-only requests + a broadcast to
+  mempool.space.
 
-If you're seeing this, you've probably already done this step. Congrats!
+Built with the modern audited [`@scure`](https://github.com/paulmillr/scure-btc-signer)
+/ [`@noble`](https://github.com/paulmillr/noble-hashes) libraries. Pure JS: no
+`crypto.subtle`, so it works from the filesystem with no server.
 
-```bash
-# create a new project in the current directory
-npx sv create
+## Develop
 
-# create a new project in my-app
-npx sv create my-app
-```
+Requires [Bun](https://bun.sh).
 
-## Developing
+```bash
+bun install
+bun run dev      # http://localhost:5173 (rebuilds on each refresh)
+```
 
-Once you've created a project and installed dependencies with `npm install` (or `pnpm install` or `yarn`), start a development server:
+## Build
 
 ```bash
-npm run dev
-
-# or start the server and open the app in a new browser tab
-npm run dev -- --open
+bun run build    # → dist/index.html  (one self-contained file)
 ```
 
-## Building
+`dist/index.html` inlines all code, the crypto libraries, the QR generator, and
+the CSS. **To use it offline: save that one file and open it directly in a
+browser** — no server, no internet.
 
-To create a production version of your app:
+## Using it offline (air-gapped spending)
 
-```bash
-npm run build
-```
+1. **Online device** (with internet): open the wallet, let it scan, then go to
+   **Tools → Export snapshot**. This downloads a JSON file of your UTXOs and fee
+   rates. It contains **no keys**.
+2. Move the file to your **offline device** (e.g. an air-gapped laptop running
+   the saved `index.html`).
+3. On the offline device, open the wallet, enter your **seed phrase** with
+   *Offline mode* checked, then **Tools → Import snapshot**.
+4. Build and **sign** a transaction. Copy/download the signed hex (or scan the
+   QR) and broadcast it from any online device (e.g. mempool.space → Broadcast).
+
+## Layout
+
+| File | Purpose |
+| --- | --- |
+| `src/wallet.js` | BIP84 derivation, scanning, coin selection, signing, snapshots |
+| `src/api.js` | mempool.space REST wrapper (the single network choke-point) |
+| `src/app.js` | UI controller (vanilla DOM) |
+| `src/qr.js` | QR → SVG (zero-dep) |
+| `src/format.js` | sat/BTC formatting helpers |
+| `src/style.css` | Hand-rolled styles |
+| `build.js` / `dev.js` | Bun bundler → single inlined `index.html`, and dev server |
 
-You can preview the production build with `npm run preview`.
+## Security notes
 
-> To deploy your app, you may need to install an [adapter](https://svelte.dev/docs/kit/adapters) for your target environment.
+- This is self-custody software handling real keys — review the code before
+  trusting it with funds, and prefer an offline machine for the seed phrase.
+- Mainnet and testnet are both supported (select on the unlock screen).
+- The seed phrase and passphrase live only in memory while the page is open;
+  locking or closing the tab clears them.