refactor(tui): split app.rs and views.rs into modular components

[daisuke8000]

Summary

• Split app.rs (1165 lines) into app/ directory with 4 modules
• Split views.rs (1004 lines) into views/ directory with 4 modules
• Additional splits for content/mod.rs functionality into 9 modules

Changes

app/ directory

File	Responsibility
mod.rs	App struct, initialization, rendering
action_handler.rs	AppAction event processing
data_handler.rs	Async data loading with tokio tasks
input_handler.rs	Keyboard event processing

views/ directory

File	Responsibility
mod.rs	Module exports
welcome.rs	Welcome screen and placeholder
lists.rs	Questions, Collections, Databases list views
drill_down.rs	Collection→Questions, DB→Schemas→Tables
results.rs	Query result and table preview rendering

content/ additional modules

File	Responsibility
types.rs	ContentView, InputMode, SortOrder, QueryResultData
navigation.rs	Navigation stack and view transitions
selection.rs	Selection navigation for each view type
search.rs	Search mode handling
sort.rs	Sort modal and index-based sorting
filter.rs	Filter modal and filtering
result_search.rs	All-column search in results
pagination.rs	Pagination and scrolling
key_handler.rs	Key event handling per view

Test plan

• All 32 tests pass
• cargo fmt
• cargo clippy (1 warning for unused trait method)
• cargo build