TASK-029 TUI: Persist last active page across restarts

Description

The TUI always opens on the Personal page. Save and restore the last active page alongside theme state.

State file location: ~/.local/share/dashboard/tui_state.json (shared with TASK-021).

Implementation:

• On action_page(), update the state file with "page_id": page_id
• On on_mount(), read the file and call action_page(saved_page_id) after layout is ready
• Validate that saved_page_id is in PAGES before applying; fall back to page-personal

Acceptance Criteria

• #1 TUI reopens on the page that was active when it was last closed
• #2 Invalid saved page falls back to Personal without error
• #3 Shares state file with theme persistence (TASK-021)