Add configurable logTimeFormat option (#515, #427)

[dolonet]

What

Adds a logTimeFormat config option so the log timestamp format is
configurable instead of hardcoded to Unix milliseconds.

internal/cli/run_proxy.go previously did
zerolog.TimeFieldFormat = zerolog.TimeFormatUnixMs unconditionally.
That value carries no timezone, so on plain log viewers (podman logs,
logread on OpenWRT) it reads as UTC and the host TZ is ignored —
exactly what #515 reports, and the long-standing ask in #427.

Approach

Based on the approach of #531 by @resolvicomai — same field name
(logTimeFormat), same preset set, default unix-ms, Go-layout
passthrough. I reworked it to address the review nit on that PR
(a bare string field) by giving it a proper typed wrapper, matching
how every other config field is modeled, and added focused tests.
Credit for the original shape is theirs.

This matches the design you approved on #515.

Details

• New TypeLogTimeFormat wrapper in internal/config, modeled on the
  existing typed fields (Set/Get/UnmarshalText/String), plus a
  ZerologFormat() resolver that maps presets to zerolog constants.
• Presets: unix, unix-ms, unix-micro, unix-nano, rfc3339,
  rfc3339-nano. Anything else is passed through as a Go reference-time
  layout.
• Default is unix-ms → zerolog.TimeFormatUnixMs, so the current
  log output is byte-for-byte unchanged unless the option is set.
• Set() rejects an empty value (fail-fast on config load). Go layouts
  are permissive strings, so validation beyond "known preset or
  non-empty" isn't reliably possible — documented on the type.
• example.config.toml documents the presets; all example values
  render the same instant (2026-05-20T12:23:45.123Z).

Omitted: iso8601 (always-UTC)

zerolog formats the layout against the timestamp's own location; there's
no native always-UTC constant, and forcing it would need a custom
TimestampFunc/hook. I left it out rather than ship something fragile —
the numeric presets already give an always-UTC option, and rfc3339
gives the TZ-aware one. Happy to add it if you'd like a specific shape.

Tests

• Config parsing: each preset parses; default is unix-ms; empty value
  behaves as unset→default (consistent with prefer-ip and the other
  optional string fields); Set() rejects empty at the type level.
• Behavior: makeLogger sets zerolog.TimeFieldFormat to the resolved
  preset/layout, including the unchanged default.
• No network/httpbin-dependent tests.

go build ./..., go test ./internal/... -race, and
golangci-lint run (v2) are all clean locally.

Ready for your review, no rush.

Fixes #515.
Resolves #427.

[alexeys85]

It would be nice to have an option to output log to file.