feat: improve responsive terminal UI

[shawnsw]

Summary

Improve responsive terminal layout for compact panes.

• Add compact Work / Usage / System tabs
• Improve section sizing at narrow widths
• Add keyboard and mouse navigation for compact views
• Restrict click actions to left-click only

Verification

• cargo test

[graykode]

lgtm — security clean (UI-only diff, no new deps/network/shell, mouse capture is the standard TUI tradeoff). solid responsive split: NarrowTab/NarrowSection in App with clamp_* keeping state coherent across panel toggles, the <DESKTOP_WIDTH branch handles tabs/sections/zoom + mouse + keyboard + scroll, and the new compact tests pin both rendered text and click targets so future width-threshold tweaks won't silently regress hit-testing. nice incidental cleanup moving the sessions.rs test module to end-of-file (fixes the pre-existing items_after_test_module warning). also confirmed merging current main and running cargo build / cargo test / cargo clippy --all-targets -- -D warnings is all green (133/133, 0 lint warnings) on top of the merge.

couple of follow-up nits, none blocking:

• src/ui/mod.rs::session_at duplicates the detail_reserve decision tree from src/ui/sessions.rs::draw_sessions_panel (the if app.show_timeline ... else if inner.height <= 12 ... else ... ladder). same story for ports_kill_at mirroring draw_ports_panel's row layout (header + live + orphan + kill). these have to stay in lockstep for click hit-testing to land on the right row, and right now nothing structurally enforces that. extracting the row-layout decisions into a helper used by both renderer and hit-tester would prevent silent drift the next time someone adjusts column logic.
• mouse capture disables the terminal's native text selection for most terminals (users have to hold Shift). worth a one-liner in README or ? help so new users know.
• set_narrow_tab(tab) silently no-ops when the tab has no visible panels, so pressing w/u/s on a config where that group is fully hidden does nothing. small UX surprise; a flash status message ("no panels in this tab") would smooth it. happy to land as a follow-up.