Description

Theme resets to flexoki every time the TUI starts. Save the selected theme index to a local state file so the last-used theme is restored on next launch.

State file location: ~/.local/share/dashboard/tui_state.json (create dir if needed).

Implementation:

• On action_next_theme(), write {"theme_idx": N, "page_id": "..."} to the state file
• On on_mount(), read the file if it exists and restore _theme_idx before applying the theme
• Use json + pathlib.Path; silently ignore missing or malformed state file
• A single state file shared with TASK-022 (page persistence)

Acceptance Criteria

• #1 Selected theme persists after quitting and relaunching the TUI
• #2 Missing or corrupt state file falls back to flexoki without error
• #3 State file is human-readable JSON