Add browser-only CSV workflow, UI overhaul, and feature roadmap

[onshisan]

Motivation

• Provide a fully in-browser workflow that avoids external ILS/API dependencies so staff can generate temporary cards from a local CSV export.
• Improve usability by adding search, selection, and a live card preview for faster issuance and printing.
• Document alternative directions and priorities for future browser-first features such as batch printing, sequential generation, and thermal/ESC-POS options.

Description

• Update README.md to describe the in-browser CSV workflow, supported CSV headers, printing guidance, and a brief note about thermal printing options.
• Add docs/BROWSER_FEATURE_ROADMAP.md containing a prioritized, detailed roadmap for browser-only enhancements and thermal printing architectures.
• Replace the single-purpose HTML with a polished libcardgen.html containing a responsive UI, CSS variables, accessible controls, a patron file loader, name search, manual barcode input, live preview, and print button.
• Replace the original script.js with a stateful implementation that parses CSV (parseCsv), normalizes/searches names, loads/clears patron data, selects patrons to populate the barcode, validates numeric barcodes, and renders barcodes using JsBarcode.

Testing

• No automated tests were added or executed for this change.

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Codex Task