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/semver/range.bnf

@@ -0,0 +1,16 @@
+range-set  ::= range ( logical-or range ) *
+logical-or ::= ( ' ' ) * '||' ( ' ' ) *
+range      ::= hyphen | simple ( ' ' simple ) * | ''
+hyphen     ::= partial ' - ' partial
+simple     ::= primitive | partial | tilde | caret
+primitive  ::= ( '<' | '>' | '>=' | '<=' | '=' ) partial
+partial    ::= xr ( '.' xr ( '.' xr qualifier ? )? )?
+xr         ::= 'x' | 'X' | '*' | nr
+nr         ::= '0' | [1-9] ( [0-9] ) *
+tilde      ::= '~' partial
+caret      ::= '^' partial
+qualifier  ::= ( '-' pre )? ( '+' build )?
+pre        ::= parts
+build      ::= parts
+parts      ::= part ( '.' part ) *
+part       ::= nr | [-0-9A-Za-z]+