fix: UI/UX polish

[datlechin]

Summary

Comprehensive UI/UX polish pass addressing 17 issues found during a full app audit.

Critical fixes

• Empty result state for SELECT queries returning 0 rows
• Import button disabled when file preview fails (encoding mismatch)
• License activation load error now shown to user (was silent)

High-priority fixes

• Execution indicator shows "Executing..." text for non-ClickHouse databases
• Pagination shows loading spinner while fetching pages
• Filter preset save checks for duplicate names, auto-appends suffix
• AI model fetch error now has a Retry button
• Connection test shows "Connected" label (not just a fading icon)
• "Reset All Settings to Defaults" button in General settings

Medium fixes

• Onboarding feature descriptions localized via String(localized:)
• Column header tooltips show column type on hover
• "Copy Error" button on inline error banner
• "Format SQL" added to editor right-click context menu
• App icon shadow uses system accent color instead of hardcoded orange
• Sync tab labeled "(Pro)" in Settings
• Export format picker shows brief descriptions per format

Test plan

• Run a SELECT that returns 0 rows — verify "No rows returned" state
• Open Import with wrong encoding — verify Import button is disabled
• Test connection — verify "Connected" label appears on success
• Right-click in SQL editor — verify "Format SQL" menu item works
• Navigate pagination — verify spinner shows during load
• Check column headers — verify tooltip shows column type
• Open Settings > General — verify "Reset All Settings" button with confirmation
• Save a filter preset with duplicate name — verify auto-dedup