fix: P0 Windows Edge path, add time localization, show --output, cleanup tech debt

- Fix auth.py subprocess script Windows Edge cookie path inconsistency
- Add timeutil.py for UTC→local time and relative time conversion
- Integrate time localization into formatter.py and serialization.py
- Add --output/-o option to show command for saving tweet detail as JSON
- Remove constants.py legacy aliases (USER_AGENT, SEC_CH_UA)
- Remove client.py backward-compat delegation methods and re-exports
- Update test imports to use parser module directly

twitter_cli/timeutil.py

@@ -0,0 +1,71 @@
+"""Time formatting utilities for twitter-cli.
+
+Converts Twitter API timestamps (e.g. "Sat Mar 08 12:00:00 +0000 2026")
+into human-friendly local time and relative time strings.
+"""
+
+from __future__ import annotations
+
+import logging
+from datetime import datetime, timezone
+from typing import Optional
+
+logger = logging.getLogger(__name__)
+
+# Twitter API timestamp format: "Sat Mar 08 12:00:00 +0000 2026"
+_TWITTER_TIME_FORMAT = "%a %b %d %H:%M:%S %z %Y"
+
+
+def _parse_twitter_time(created_at: str) -> Optional[datetime]:
+    """Parse a Twitter API timestamp into a timezone-aware datetime."""
+    if not created_at:
+        return None
+    try:
+        return datetime.strptime(created_at, _TWITTER_TIME_FORMAT)
+    except (ValueError, TypeError):
+        logger.debug("Failed to parse Twitter timestamp: %s", created_at)
+        return None
+
+
+def format_local_time(created_at: str) -> str:
+    """Convert Twitter timestamp to local time string.
+
+    Returns "2026-03-14 21:08" or the original string on parse failure.
+    """
+    dt = _parse_twitter_time(created_at)
+    if dt is None:
+        return created_at
+    local_dt = dt.astimezone()
+    return local_dt.strftime("%Y-%m-%d %H:%M")
+
+
+def format_relative_time(created_at: str) -> str:
+    """Convert Twitter timestamp to a relative time string.
+
+    Returns e.g. "2m ago", "3h ago", "5d ago", or the original string on failure.
+    """
+    dt = _parse_twitter_time(created_at)
+    if dt is None:
+        return created_at
+    now = datetime.now(timezone.utc)
+    delta = now - dt
+    seconds = int(delta.total_seconds())
+
+    if seconds < 0:
+        return "just now"
+    if seconds < 60:
+        return "%ds ago" % seconds
+    minutes = seconds // 60
+    if minutes < 60:
+        return "%dm ago" % minutes
+    hours = minutes // 60
+    if hours < 24:
+        return "%dh ago" % hours
+    days = hours // 24
+    if days < 30:
+        return "%dd ago" % days
+    months = days // 30
+    if months < 12:
+        return "%dmo ago" % months
+    years = days // 365
+    return "%dy ago" % years