feat(onboarding): expand welcome tour guide with 6 interactive steps and shortcuts matrix (#606)

[ZhuchkaTriplesix]

Summary

Expands the interactive Welcome Tour dialog to 6 comprehensive steps, introducing styled platform-aware visual keycap badges (KbdBadge) and a categorized keyboard shortcuts cheatsheet matrix.

Changes

1. Expanded 6-Step Guide Flow (lib/features/onboarding/welcome_tour_dialog.dart):
   • Step 1 (Connections): PostgreSQL, MySQL, SQLite, Redis, MongoDB, Extension drivers + Ctrl+N / Cmd+N.
   • Step 2 (Fluid Sidebar & Navigation): Schema tree navigation, object context menus, and Ctrl+B / Cmd+B fluid collapse.
   • Step 3 (Pro SQL Editor & Workspaces): Query execution (F5 / Ctrl+Enter), multi-tab workspaces, scripts save/open, and history.
   • Step 4 (Interactive 2D Grid & DML Staging): In-place cell editing, Shift+Click range selection, TSV copy (Ctrl+C), and DML review/apply.
   • Step 5 (Compound Filter Bar & Quick Calc): Live query autocomplete filter bar (Ctrl+F), Groupings/Pivot drawer (Ctrl+G), and status bar aggregates (Sum, Avg, Min, Max, Median).
   • Step 6 (Hotkeys Matrix & Demo Playground): Full categorized keyboard shortcut cheatsheet with KbdBadge keycaps + 1-click Demo Playground launcher.
2. Visual Keycap Badge Component:
   • Added KbdBadge with OS-aware keycap styling (Cmd for macOS, Ctrl for Windows/Linux) and soft shadows.
3. Tests:
   • Updated widget tests in test/features/onboarding/welcome_tour_dialog_test.dart verifying full 6-step slide progression, hotkeys view, skip/home actions, and demo launcher.

Closes #606