refactor: complete Phase 13 deep architecture refactoring (etaps 3-7)

Split monolithic files into modular architecture:
- ui/messages.rs (893→365 lines): extract modals/, compose_bar.rs
- tdlib/messages.rs (836→3 files): split into messages/mod, convert, operations
- config/mod.rs (642→3 files): extract validation.rs, loader.rs
- Code duplication cleanup: shared components, ~220 lines removed
- Documentation: PROJECT_STRUCTURE.md rewrite, 16 files got //! docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ui/modals/mod.rs

@@ -0,0 +1,17 @@
+//! Modal dialog rendering modules
+//!
+//! Contains UI rendering for various modal dialogs:
+//! - delete_confirm: Delete confirmation modal
+//! - reaction_picker: Emoji reaction picker modal
+//! - search: Message search modal
+//! - pinned: Pinned messages viewer modal
+
+pub mod delete_confirm;
+pub mod reaction_picker;
+pub mod search;
+pub mod pinned;
+
+pub use delete_confirm::render as render_delete_confirm;
+pub use reaction_picker::render as render_reaction_picker;
+pub use search::render as render_search;
+pub use pinned::render as render_pinned;