Adapt TUIs to light and dark terminal backgrounds

[skarim]

The submit, view, and modify TUIs were tuned for dark terminals. On light
or solarized-light backgrounds the result was hard to read and inverted:
primary text used ANSI white (invisible on white), dim chrome used light
grays (too faint), and accents used bright cyan (low contrast) — so
"active" things looked lighter than "disabled" ones.

Introduce a centralized, background-aware color palette and migrate all
three TUIs to it:

• internal/tui/shared/theme.go: a semantic palette of lipgloss.AdaptiveColor
  values (primary/muted/faint text, chrome/border, accent, PR-state colors,
  badge backgrounds, row shade, button, switch). lipgloss resolves the
  light/dark variant per render from the terminal background, which Bubble
  Tea detects at startup; terminals that don't report it fall back to dark,
  preserving the original look.
• Replace every hardcoded ANSI color in shared/, submitview/, and
  modifyview/ with palette roles. The four pre-rendered status icons now
  render at use-time so their adaptive colors resolve correctly. The submit
  markdown preview picks glamour's light or dark style from the detected
  background.
• GH_STACK_THEME=auto|light|dark forces the palette for terminals that
  mis-detect (some SSH/tmux setups); wired via the root command's
  PersistentPreRun before any render. Documented in the README and CLI docs.

Neutral text/chrome use truecolor hex (GitHub Primer-inspired) for
predictability across themes, including solarized which repurposes ANSI
8-15; lipgloss downsamples on terminals without truecolor.

Tests verify the palette resolves differently for light vs dark and that
GH_STACK_THEME is honored.

---

_{Stack created with GitHub Stacks CLI • Give Feedback 💬}

[Copilot]

Pull request overview

This PR makes the submit, view, and modify interactive TUIs readable on both light and dark terminal backgrounds. Previously colors were hardcoded ANSI indices tuned only for dark terminals (e.g. ANSI white was invisible on light backgrounds, accents were low-contrast). It introduces a centralized, background-aware palette of lipgloss.AdaptiveColor roles and migrates every hardcoded color across shared/, submitview/, and modifyview/ to it.

Changes:

• New internal/tui/shared/theme.go defines a semantic, GitHub Primer-inspired palette (text/muted/faint, border, accent, PR-state, badge, button, switch) using lipgloss.AdaptiveColor, plus ApplyThemeOverride() honoring GH_STACK_THEME=auto|light|dark, wired via the root command's PersistentPreRun.
• Replaces hardcoded ANSI colors with palette roles everywhere; status icons now render at use-time so adaptive colors resolve against the detected background, and the markdown preview picks glamour's light/dark style from lipgloss.HasDarkBackground().
• Adds tests (theme_test.go) verifying the palette differs by background and that GH_STACK_THEME is honored, plus README and CLI docs.

Show a summary per file

File	Description
internal/tui/shared/theme.go	New adaptive palette + ApplyThemeOverride() for GH_STACK_THEME.
internal/tui/shared/theme_test.go	Tests palette background-awareness and theme override.
internal/tui/shared/styles.go	Migrates branch/PR/diff/connector/header styles to palette; icons rendered at use-time.
internal/tui/shared/render.go	StatusIcon renders glyph+style at use-time instead of pre-rendered icons.
internal/tui/shared/header.go	Disabled-shortcut style uses ColorTextFaint.
internal/tui/submitview/styles.go	State fg/bg, chrome, switch, segment styles moved to palette; Color() returns TerminalColor.
internal/tui/submitview/screen.go	Branch circle/name/checkbox + left panel border use palette.
internal/tui/submitview/editor.go	Field/desc box borders use palette accent/border.
internal/tui/submitview/render.go	Chrome + "new PRs" highlight use palette.
internal/tui/submitview/preview.go	Selects light/dark glamour style from detected background.
internal/tui/submitview/help.go	Help overlay styles use palette.
internal/tui/modifyview/styles.go	Action/connector/status/help/transient styles use palette.
internal/tui/modifyview/model.go	Pending-summary icon uses ColorYellow.
cmd/root.go	PersistentPreRun applies the theme override before any render.
README.md, docs/.../cli.md	Document GH_STACK_THEME and auto-adaptation behavior.

Review details

• Files reviewed: 16/16 changed files
• Comments generated: 0
• Review effort level: Medium