tui: one malformed custom theme file silently disables all custom themes

Description

A single invalid themes/*.json file anywhere in the discovered .opencode directories disables every custom theme, not just the invalid one.

In packages/tui/src/context/theme.tsx, discoverThemes() parses each matched file with JSON.parse and no per-file error handling, so the first malformed file rejects the whole discovery pass. The rejection is then swallowed in syncCustomThemes() via catch(() => setStore("active", "opencode")): nothing is logged, previously working themes disappear from the picker, and the active theme silently resets to the default.

This diverges from the convention the tui config loader itself documents ("every broken-config path degrades gracefully rather than crashing" - see packages/opencode/src/config/tui.ts), where an unreadable or invalid file is skipped with a warning while the remaining valid files still apply.

Suggested direction: isolate each file's parse, skip and warn on failure instead of failing the entire batch.

Steps to reproduce

1. Create a valid custom theme at .opencode/themes/my-theme.json, select it in the TUI - works.
2. Add a second file .opencode/themes/broken.json whose content is just { (invalid JSON).
3. Restart the TUI (or trigger a theme refresh).
4. Both custom themes are now gone from the theme list and the active theme has silently fallen back to opencode. Fixing broken.json brings everything back.

Operating System

Windows 11 (the logic is platform-independent)