[Feature] Add parking fee calculator

[yeleepark]

Summary

Add a Parking Fee Calculator for estimating parking costs from entry time, exit time, and fee rules.

Why

Parking fee calculations are common in daily life, travel, events, venues, and small business operations. A local calculator can help users quickly estimate costs without manually counting time blocks or applying fee caps.

Proposed scope

• Enter entry and exit date/time
• Configure base free time, base fee, interval length, and interval fee
• Support daily maximum fee where applicable
• Support simple overnight or multi-day parking calculations
• Show total parked duration and estimated fee
• Copy the result summary to the clipboard
• Process everything locally in the browser

UX notes

• Keep the fee rule form explicit to avoid hidden assumptions
• Show a plain-language fee breakdown when possible
• Consider presets for common rules after the MVP
• Suggested microcopy: // the meter, explained

Acceptance criteria

• Entry and exit times produce a correct duration
• Configured fee rules produce a clear estimated total
• Invalid time ranges and missing fee values are handled gracefully
• Result summaries can be copied to the clipboard
• No input data is uploaded to a server