refactor: replace var with let and const for variable declarations across multiple files

[pascalre]

• Updated variable declarations from var to let or const in feed-validate.ts, publish.ts, receive.ts, reply.ts, request.ts, semp-connection.ts, chalkUtils.ts, checkparams.ts, config.ts, defaults.ts, init.ts, listfeeds.ts, logger.ts, parse.ts, prettify.ts, shorthash.ts, visualize.ts.
• This change improves code readability and maintains block scope for variables.

[gvensan]

Thanks for taking this up.

[gvensan]

The src/ changes look good to me - I verified the whole
diff is mechanical (386 pure keyword swaps plus the as const cleanups),
tsc is clean, and the build runs.

Two things before we merge:

1. Could you revert the public/ directory? The autofix stripped upstream
   eslint-disable directives out of jquery.js, jquery.slim.js, adminlte.js
   and MQTTSolaceClient.js. Those are vendored third-party files and we want
   them byte-identical to upstream so future version bumps stay clean.

2. The underlying cause is that eslint.config.mjs has no ignores block, so
   the linter walks into vendored assets. Could you add:

   { ignores: ['public/', 'dist/', 'bin/**'] }

   as the first entry in defineConfig()? That also drops the total error
   count from ~5,360 to ~1,427, which makes the remaining work much easier
   to see.

Minor: a few src/ files (config.ts, prettify.ts, shorthash.ts) have trailing
whitespace left where disable-comments were removed.