docs: refresh README with full engine + feature coverage

[broisnischal]

Brings the README in line with what the app actually ships today.

What changed

• Supported databases — added the three engines that were supported but undocumented: ClickHouse, DuckDB, and Microsoft SQL Server. Split the list into core engines vs one-click providers, and added Prisma Postgres to the providers (alongside Neon, Supabase, PlanetScale).
• New feature sections — documented capabilities that were missing entirely:
  • Live mode (real-time table change streaming)
  • Global search
  • EXPLAIN plans in the SQL console
  • Relation tree (foreign-key drill-down)
  • Schema-object creation dialogs (tables, schemas, sequences, triggers, enums, FKs) + enum management
• Backup & restore — corrected to describe the real behavior: a SQL dump with granular include toggles (schema/data/views/triggers/functions/sequences/enums), per-table selection, a live progress log, and cancellation — for PostgreSQL, MySQL, SQLite, and Cloudflare D1. (The old text claimed "SQL dumps or CSV" import, which isn't how it works.)
• Storage — noted OS-keychain credential storage; Charts now mentions ECharts + choropleth maps.
• Connecting — expanded to cover every engine and provider (DuckDB file, ClickHouse TLS, SQL Server, provider sign-in).

Verification

Every addition was checked against the codebase — engines from ActiveConnection (connection.rs), providers from providers.js, export formats from export.js, and features from the component set. No aspirational claims.

Docs-only; no release label (nothing to build/publish).

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Latest commit: 3e6c523

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[broisnischal]

Superseded by #49 (consolidated release PR, labeled release:minor → v1.12.0). All changes from this PR are included there.