feat: TUI mode with scroll, mouse, conversation rendering fixes

[PTFOPlayer]

• Add full TUI mode with conversation pane, input bar, status bar, sidebar
• Fix scroll: mouse scroll and PageUp/PageDown/Home/End now always go to conversation widget regardless of focus
• Fix mouse scroll: SGR mouse event parsing was broken (< prefix not stripped from parameter string, causing scroll events to be parsed as left-click)
• Fix conversation text overflow: text now wraps at conversation area width (area.x + area.width - 1) instead of full screen width, preventing overflow into sidebar area
• Fix conversation line height: now uses character-level wrapping matching actual screen rendering, preventing visual rows from being undercounted
• Fix scroll offset: changed from conversation-line units to visual-row units that account for text wrapping, so scrolling works correctly with long messages
• Fix partially-scrolled line rendering: added write_str_wrapped_skip_clipped to Screen that skips N visual rows before rendering, so lines scrolled off the top of the viewport don't re-render their full text
• ToolResult now renders all content lines (not just first), with background fill, line truncation, and a 20-line cap with truncation indicator
• ToolResult trims trailing blank lines from content
• Scrollbar column is reserved (1 column) so text doesn't overlap it
• Add thinking indicator: shows [thinking] with spinner during thinking phase, accumulates thinking text, transitions to 'Responding' label on first streaming text
• Widget trait render method changed from &self to &mut self
• All widget implementations updated accordingly