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/reaction_picker.rs

@@ -0,0 +1,13 @@
+//! Reaction picker modal
+
+use ratatui::{Frame, layout::Rect};
+
+/// Renders emoji reaction picker modal
+pub fn render(
+    f: &mut Frame,
+    area: Rect,
+    available_reactions: &[String],
+    selected_index: usize,
+) {
+    crate::ui::components::render_emoji_picker(f, area, available_reactions, selected_index);
+}