docs: enhance README (badges, quickstart, positioning)

[stukenov]

Summary

OSS marketing pass on the README — enhances presentation and positioning while preserving all existing content and voice. No functionality or commands invented; every command maps to a real package.json script.

Changes

• Badge row: added a TypeScript and a "Built with Tauri" badge alongside the existing license / platform / version badges. (No npm badge — keste is a Tauri desktop app and is not published to npm, confirmed via npm view keste → 404.)
• Value prop: added a concise one-line tagline under the title summarizing keste as a portable, scriptable, LLM-friendly SQLite spreadsheet.
• Screenshot/demo placeholder: added a note inviting a screenshot/GIF (no fabricated image link).
• Why Keste: new positioning section emphasizing SQLite-as-file-format, SQL-native access, scriptability, and LLM/AI-workflow friendliness — bridging spreadsheets and SQL.
• Quickstart: expanded with the real, already-present scripts — web-only npm run dev, dedicated npm run tauri:dev / npm run tauri:build equivalents, and a "Running Tests" block (npm test, test:ui, test:coverage). All verified to exist in package.json.

Context

Part of a portfolio marketing pass to improve the repo's first impression and discoverability. Documentation only — no code changes, LICENSE untouched.

🤖 Generated with Claude Code