fix: move hashcash PoW from build artifact to JSX source

The hashcash proof-of-work implementation was incorrectly added to the
build artifact web/dist/app.js instead of the source file web/src/app.jsx.
Running web/build.sh would overwrite all hashcash changes.

Changes:
- Add checkLeadingZeros() and mintHashcash() functions to app.jsx
- Integrate hashcash into LoginScreen: fetch hashcash_bits from /server,
  compute stamp via Web Crypto API before session creation, show
  'Computing proof-of-work...' feedback
- Remove web/dist/ from git tracking (build artifacts)
- Add web/dist/ to .gitignore

.gitignore

@@ -36,3 +36,4 @@ data.db
 debug.log
 /neoirc-cli
 web/node_modules/
+web/dist/