Theme: add a light terminal theme

[burakgon]

Why: The app ships two dark themes (dark, oled) — great on OLED phones, but there's no light option for bright-daylight use, and no "follow the system" mode.

Where:

• packages/web/src/pwa/theme.ts — extend ThemeName to "dark" | "oled" | "light", add TERMINAL_BG + THEME_COLOR entries, keep the stored-value validation pattern.
• packages/web/src/styles/tokens.css — add a [data-theme="light"] block overriding the surface palette (ink → paper, text colors flipped, keep the coral accent readable: check contrast ≥ AA).
• packages/web/src/settings/SettingsPanel.tsx — add the option to the existing theme picker.
• packages/web/src/chat/TerminalView.tsx — xterm theme colors come from TERMINAL_BG; give light mode a sensible xterm palette (background, foreground, selection).

Acceptance criteria:

• Three-way choice in Settings; applies instantly (no reload) including the xterm canvas and the <meta name="theme-color">.
• Stored value survives reload; unknown stored values fall back to dark (pattern already in loadTheme).
• Terminal text meets WCAG AA contrast in light mode.
• Tests: extend packages/web/src/pwa/theme.test.ts for the new value + fallback.

Stretch (optional, separate PR welcome): a fourth "system" option that follows prefers-color-scheme.