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

[ZhuchkaTriplesix]

Motivation & Goal

Enhance the interactive Welcome Tour dialog (lib/features/onboarding/welcome_tour_dialog.dart) to thoroughly educate users on Querya Desktop capabilities and empower them with essential keyboard shortcuts for high-efficiency workflow.

Proposed Scope

1. 6-Step Interactive Guide Flow:

   • Step 1 (Connections): Direct database drivers (Postgres, MySQL, SQLite, Redis, MongoDB, Extensions) + Ctrl+N / Cmd+N shortcut.
   • Step 2 (Sidebar & Shell): Schema tree navigation, context menus, and Ctrl+B / Cmd+B fluid sidebar collapse.
   • Step 3 (SQL Editor): Query execution (F5 / Ctrl+Enter / Ctrl+R), tabs (Ctrl+Shift+N), and file management (Ctrl+S / Ctrl+O).
   • Step 4 (Data Grid & DML): Inline editing, cell range selection (Shift+Click), TSV copy (Ctrl+C), and DML review & apply (Ctrl+S).
   • Step 5 (Filter & Pivot): Compound query filter bar (Ctrl+F), autocomplete, Groupings/Pivot panel (Ctrl+G), and Quick Calc stats.
   • Step 6 (Hotkeys Matrix & Playground): Full categorized keyboard shortcut cheatsheet with styled visual keycaps (KbdBadge) and 1-click Demo Playground launcher.

2. Visual Keycap Badges:

   • Implement clean, responsive keyboard badge components for Ctrl, Cmd, Shift, Enter, F5, B, F, G, etc. adapting to macOS (Cmd) and Windows/Linux (Ctrl).

3. Global Guide Access:

   • Ensure F1 and Ctrl+Shift+H / Cmd+Shift+H toggle the tour from anywhere in the application.

4. Testing:

   • Update and add widget tests in test/features/onboarding/welcome_tour_dialog_test.dart validating slide transitions, shortcut badges, and demo launch action.