Add multiple editor tabs with independent connections (#14)

[muk2]

Summary

• Refactors App to support multiple editor tabs via a new EditorTab struct, each with its own editor buffer, database connection, query results, sidebar state, and query history
• Adds a tab bar (visible when 2+ tabs exist) showing connection status indicator and database name for each tab
• Supports keyboard shortcuts: Ctrl+T (new tab), Ctrl+W (close tab), Alt+Left/Right (cycle tabs), Alt+1..9 (jump to tab N)

Changes

• src/ui/app.rs: Created EditorTab struct encapsulating per-tab state; refactored App to hold Vec<EditorTab> and active_tab; added tab()/tab_mut() accessors; added new_tab()/close_tab()/next_tab()/prev_tab() methods; updated all input handlers and business logic to use tab-based state
• src/ui/components.rs: Added draw_tab_bar() function; updated layout to conditionally include tab bar row; updated all rendering functions to read from app.tab() instead of direct app.* fields; added tab shortcuts to help overlay; added tab position to status bar

Design Decisions

• Per-tab state extracted into EditorTab to cleanly separate concerns
• Connection dialog remains shared (one dialog at a time) but connects to the active tab
• Tab bar only shows when there are 2+ tabs to avoid wasting space in single-tab mode
• Closing the last tab replaces it with a fresh empty tab (never zero tabs)
• Esc in connection dialog for an unconnected tab with other tabs open closes the tab instead of quitting

Test plan

• cargo check passes
• cargo clippy passes with no warnings
• cargo fmt produces no changes
• All 124 tests pass
• Manual testing: Ctrl+T creates new tab with connection dialog
• Manual testing: Connect different databases in different tabs
• Manual testing: Alt+Left/Right and Alt+1..9 switch tabs
• Manual testing: Ctrl+W closes tab, last tab creates fresh one
• Manual testing: Tab bar shows correct connection indicators

Closes #14

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