Refactor app.rs monolith into focused submodules

[muk2]

Summary

• Splits the ~1900-line src/ui/app.rs monolith into 5 focused submodules under src/ui/app/
• Pure refactoring with zero behavioral changes - all 225 existing tests pass unchanged
• Each module handles a single concern, improving navigability and maintainability

Module Structure

src/ui/app/
├── mod.rs          (527 lines) Core App struct, types, enums, constants, new(), tick()
├── connection.rs   (412 lines) Connection dialog input, start_connect, finish_connect
├── sidebar.rs      (221 lines) Sidebar navigation, table inspector, schema refresh
├── editor.rs       (499 lines) Editor input, query execution, autocomplete, query parsing
└── results.rs      (282 lines) Results navigation, table inspector input, export

What moved where

Method/Handler	From	To
handle_connection_dialog_input, start_connect, finish_connect, try_auto_connect	app.rs	connection.rs
handle_sidebar_input, handle_sidebar_select, open_table_inspector, refresh_schema	app.rs	sidebar.rs
handle_editor_input, execute_query, update_autocomplete, find_query_boundaries	app.rs	editor.rs
handle_results_input, handle_export_input, handle_table_inspector_input, perform_export, copy_selected_cell	app.rs	results.rs
All type definitions, App struct, new(), handle_input(), tick(), utility methods	app.rs	mod.rs

Design Decisions

• Used Rust's split impl block pattern - each submodule adds methods to App via impl App {}
• Helper methods use pub(super) or pub(crate) visibility to keep the public API unchanged
• Module-private helpers (like dialog_field_len) stay private to their module
• ui/mod.rs still uses mod app; pub use app::*; - no changes needed to external imports

Test plan

• cargo build compiles without warnings
• cargo test - all 225 tests pass (zero behavioral changes)
• Verified all public types and functions are still exported correctly

Closes #39

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