tui: copy chat to system clipboard via reading mode + selection fixes ...

Two ways to copy chat content out of the TUI now work:

1. In-app selection (Ctrl+C, OSC 52): _ChatItemView.render() now builds a
   selection-aware Content from the rich renderable instead of letting Static
   wrap a Panel into a non-selection-aware RichVisual — so the magenta
   drag-select highlight actually renders and get_selection returns clean,
   chrome-free text. rich Segment styles are converted to Textual Style
   (Style.from_rich_style) so the selection_style merge doesn't crash, and
   None segment styles are guarded. get_content_height is overridden to
   compute auto-height straight from the rich renderable; without it the
   width-keyed Content churned new Visual objects per width change and spread
   the layout-settling cascade across refreshes, leaving a deferred scroll_end
   at an intermediate max_scroll_y (chat no longer stuck to bottom). Theme
   defines screen-selection-background/foreground so the highlight is visible.

2. Reading mode (Ctrl+R) for terminals without OSC 52 (e.g. GNOME Console,
   TERM_PROGRAM=kgx, which silently ignores OSC 52 so Ctrl+C never reaches the
   OS clipboard). Toggling the app's reading-mode class hides the chrome around
   the chat (right status/todo column, input prompt, status bar, chat outer
   border) and re-renders every message without its Panel borders via the
   registry's new render_plain — so Shift+drag + Ctrl+Shift+C copies just the
   conversation text to the system clipboard. ContentRenderer.render_plain
   defaults to unwrapping Panel/Padding(Panel) to its body; assistant messages
   and plan_ready override it to raw Text(content) so rich Markdown doesn't
   re-wrap fenced code blocks in their own bordered Panels. ChatPanel tracks
   reading mode and re-renders visible widgets in place (scroll preserved);
   new streamed items pick up the mode on mount.

Help (/help) now lists keys and documents both copy paths.

Tests: render_plain parametrized across all item types, message renderer
plain tests, chat panel reading-mode toggle (borders off/on, scroll kept,
new items borderless), tui_app reading-mode chrome hide, plus the existing
selection suite. Full suite green (922 passed, 1 skipped).

Navi Code TUI: command palette and themed markdown ...

- Add CommandPaletteScreen modal with fuzzy filtering and keyboard navigation.

- Bind Ctrl+P to push the palette; selected commands execute via registry.

- Make MarkdownRenderer theme-aware: headings, inline code, links, and code blocks use gnexus palette.

- Pick dracula/github-light code theme based on active theme brightness.

Co-Authored-By: Claude <noreply@anthropic.com>

Navi Code TUI: apply gnexus theme to widgets and renderers ...

- Register Navi themes as Textual themes so $tui-* CSS variables resolve.

- Update ChatPanel, StatusPanel, InputBox to use palette colors.

- Update all content renderers to draw from the active theme.

- Add set_active_theme/get_active_theme helpers for runtime color access.

Co-Authored-By: Claude <noreply@anthropic.com>

Navi Code TUI: extend theme with selection and link colors ...

Add selection (magenta) and link (teal) for cursor line, active selection and URLs.

Tests still green.

Co-Authored-By: Claude <noreply@anthropic.com>

Navi Code TUI: add gnexus-ui-kit color theme support ...

- Extract exact gnexus-ui-kit cyberpunk palette from
  webclient/vendor/gnexus-ui-kit/src/scss/_palette-colors.scss.
- Add clients/terminal/tui/themes.py with Theme dataclass, ThemeRegistry,
  and gnexus-dark / gnexus-light themes.
- Wire theme into NaviCodeTui via _apply_theme() on mount; default is
  gnexus-dark.

Tests: 463 passed, 1 skipped (excluded unrelated websocket test).

Co-Authored-By: Claude <noreply@anthropic.com>