feat: add libSQL / Turso database driver plugin

[datlechin]

Summary

• Add libSQL / Turso driver plugin via Hrana HTTP protocol (POST /v2/pipeline)
• Works with both Turso cloud databases and self-hosted sqld instances
• Pure URLSession transport, zero native dependencies, registry-distributed
• Fix apiOnly connection form validation to allow empty password when requiresAuthentication = false

What's included

Plugin files (Plugins/LibSQLDriverPlugin/):

• LibSQLPlugin.swift — registration class with SQLite-compatible dialect
• HranaHttpClient.swift — HTTP client with typed Hrana value decoding, cancellation, error handling
• LibSQLPluginDriver.swift — full driver: connect, query, schema introspection, DDL generation, streaming
• Info.plist — PluginKit version 5

Integration:

• Registry metadata snapshot in PluginMetadataRegistry+RegistryDefaults.swift
• Turso → libSQL type alias in PluginMetadataRegistry.swift
• CI case in build-plugin.yml (tag: plugin-libsql-v*)
• Turso SVG icon in all asset catalogs

Bug fix:

• ConnectionFormView+Helpers.swift: isValid now skips password requirement for apiOnly plugins with requiresAuthentication = false

Docs:

• New docs/databases/libsql.mdx with setup, credentials, features, troubleshooting
• Added to docs.json nav, index.mdx table, overview.mdx card grid

Closes #205

Test plan

• Build LibSQLDriverPlugin scheme — verify it compiles
• Build TablePro scheme — verify full app compiles
• Launch app, verify libSQL / Turso appears in connection type picker
• Select libSQL / Turso, verify form shows Database URL + optional token
• Verify save works with empty token (self-hosted sqld)
• Connect to Turso cloud database with URL + token
• Connect to local sqld (http://localhost:8080, no token)
• Browse tables, view structure, indexes, foreign keys, DDL
• Execute queries, verify column types are populated
• Test EXPLAIN query plan
• Test libsql:// URL auto-rewrite to https://
• Verify lint passes: swiftlint lint Plugins/LibSQLDriverPlugin/