Fix code review issues: dead code, error handling, alignment, DRY defaults

[cygnusb]

Summary

• Dead code removed: modes/tail.py (run_tail) was never called from cli.py; deleted entirely
• Config error handling fixed: Bad YAML / invalid config structure now surfaces the real error instead of silently falling through to a misleading --host is required message; adds ConfigFileNotFoundError subclass so only a missing default config path is silently ignored
• --align wired up in TUI modes: The flag was stored but never passed to MessageLogWidget; now threaded through both GraylogDashboard and LogsOnlyApp
• Active stream highlighted: StreamsWidget.update_streams accepted active_id but never applied it; now sets lv.index after populating the list
• Auth errors in throughput fetches trigger exit: _poll previously only called self.exit() for auth errors on the messages fetch; now checks all three results
• detect_color() removed: Defined in formatter.py but never imported or called
• except Exception narrowed: JSON parse errors in client.py now catch ValueError instead of bare Exception
• ThroughputWidget._data uses deque(maxlen=60): Replaces list + manual slice reassignment
• Shared default constants: DEFAULT_FIELDS, DEFAULT_RANGE_SECONDS, DEFAULT_POLL_INTERVAL_MS extracted to config.py and used across config.py, client.py, and cli.py

Test plan

• All 37 existing tests pass (uv run pytest)
• mypy clean (uv run mypy src/)
• ruff clean (uv run ruff check src/)

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