Introduce breakpoint-aware helper functions in layout.rs that
centralize per-screen responsive decisions. Each function maps a
Breakpoint to a screen-specific value, replacing scattered
hardcoded checks across view modules:
- detail_side_panel: show discussions side panel at Lg+
- info_screen_columns: 1 column on Xs/Sm, 2 on Md+
- search_show_project: hide project path column on narrow terminals
- timeline_time_width: compact time on Xs (5), full on Md+ (12)
- who_abbreviated_tabs: shorten tab labels on Xs/Sm
- sync_progress_bar_width: scale progress bar 15→50 with width
All functions are const fn with exhaustive match arms.
Includes 6 unit tests covering every breakpoint variant.
