fix: replace Codex references with OpenCode in UI strings and errors #52

Job	Run time
typecheck	32s
lint	29s
Rust tests (macOS)	1m 40s
test-js	1m 56s
Rust tests (Linux)	8m 11s
Rust tests (Windows)	12m 47s
Tauri build (${{ matrix.platform.name }})	0s
	25m 35s

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