Add a local `serve` dashboard with auto-refresh and filters

[SuvenSeo]

Goal

Add a codex-usage-tracker serve command that starts a local dashboard on 127.0.0.1.

Why

The generated static HTML works, but a local server mode would make the project feel more like a polished product and would support live Codex sessions better.

Acceptance Criteria

• codex-usage-tracker serve starts on 127.0.0.1 by default.
• The dashboard can refresh without re-running the command manually.
• Users can filter by date range, project, and model.
• The server does not expose data outside localhost by default.
• README docs include the new command and privacy boundary.