Add sync error logging, extend error flash, fix unload saves

- Log workforce sync errors to backend stdout so they appear in docker logs
- Extend error flash from 5s to 15s so errors are readable
- Replace sendBeacon (POST-only) with keepalive fetch (PUT) on unload —
  sendBeacon was hitting 404s on all three unload saves

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/node_modules/balanced-match/README.md

@@ -0,0 +1,57 @@
+# balanced-match
+
+Match balanced string pairs, like `{` and `}` or `<b>` and
+`</b>`. Supports regular expressions as well!
+
+## Example
+
+Get the first matching pair of braces:
+
+```js
+import { balanced } from 'balanced-match'
+
+console.log(balanced('{', '}', 'pre{in{nested}}post'))
+console.log(balanced('{', '}', 'pre{first}between{second}post'))
+console.log(
+  balanced(/\s+\{\s+/, /\s+\}\s+/, 'pre  {   in{nest}   }  post'),
+)
+```
+
+The matches are:
+
+```bash
+$ node example.js
+{ start: 3, end: 14, pre: 'pre', body: 'in{nested}', post: 'post' }
+{ start: 3,
+  end: 9,
+  pre: 'pre',
+  body: 'first',
+  post: 'between{second}post' }
+{ start: 3, end: 17, pre: 'pre', body: 'in{nest}', post: 'post' }
+```
+
+## API
+
+### const m = balanced(a, b, str)
+
+For the first non-nested matching pair of `a` and `b` in `str`, return an
+object with those keys:
+
+- **start** the index of the first match of `a`
+- **end** the index of the matching `b`
+- **pre** the preamble, `a` and `b` not included
+- **body** the match, `a` and `b` not included
+- **post** the postscript, `a` and `b` not included
+
+If there's no match, `undefined` will be returned.
+
+If the `str` contains more `a` than `b` / there are unmatched pairs, the first match that was closed will be used. For example, `{{a}` will match `['{', 'a', '']` and `{a}}` will match `['', 'a', '}']`.
+
+### const r = balanced.range(a, b, str)
+
+For the first non-nested matching pair of `a` and `b` in `str`, return an
+array with indexes: `[ <a index>, <b index> ]`.
+
+If there's no match, `undefined` will be returned.
+
+If the `str` contains more `a` than `b` / there are unmatched pairs, the first match that was closed will be used. For example, `{{a}` will match `[ 1, 3 ]` and `{a}}` will match `[0, 2]`.