Fix UI bugs: welcome screen, group headers, deletion nav, command palette

[shranchi0]

Summary

• Welcome screen wrong keybind: Listed k for "Kill Session" but the actual command palette key is d (Delete Session) — fixed
• Welcome screen hardcoded version: Used "v0.1.0" instead of reading from __version__ — now dynamic
• Group header ordering: _ensure_group_header always mounted headers at position 0, stacking all group headers at the top regardless of where their sessions were — now mounts before the first session in the group
• Session deletion jump: After deleting a session, always jumped to sessions[0] instead of the nearest neighbor — now prefers the next session, falls back to previous
• Unreachable actions: show_diff and set_group had action methods and default keybindings (F10/F11) but were missing from the command palette and COMMAND_MODE_MAP — added as v (Git Diff) and w (Set Group)
• Solarized hover states invisible: Both solarized dark and light themes had hover background identical to normal background — now uses distinct colors (#0a4a5c dark, #ddd6c1 light)
• ASCII group arrows: Group headers used > / v while the rest of the UI uses unicode — now uses ▶ / ▼

Test plan

• All 268 existing tests pass
• ruff check clean
• ruff format clean
• mypy clean
• Manual: open app, verify welcome screen shows correct keybinds and dynamic version
• Manual: create sessions in groups, verify headers appear above their group members
• Manual: delete a middle session, verify focus moves to neighbor not first session
• Manual: Ctrl+Space → v opens git diff, w opens group dialog
• Manual: switch to solarized dark/light theme, verify hover highlights on session items