Fix timezone handling for archive timestamps (Borg 2.x readiness)

[mr-raj12]

The problem

Archive timestamps from Borg are stored with timezone info stripped via .replace(tzinfo=None) in two locations:

• src/vorta/borg/create.py:25
• src/vorta/borg/list_repo.py:57

The existing TODO comment asks: "Keep as UTC?"

For Borg 1.x: Timestamps come without timezone suffix (already naive/local), so .replace(tzinfo=None) is a no-op. No impact today.

For Borg 2.x: Timestamps include UTC timezone info. Stripping it with .replace(tzinfo=None) stores the raw UTC value as a naive datetime, which then displays
as-is in the UI. A user in EST (UTC-5) would see a backup made at 9:24 AM local time displayed as "14:24" (the UTC value).

Display locations affected:

• src/vorta/views/archive_tab.py:289-290 — .strftime('%Y-%m-%d %H:%M')
• src/vorta/views/extract_dialog.py:115 — direct string conversion

Related: #185 flagged broader utcnow vs now inconsistency across models.

Requested Solution

Convert timezone-aware timestamps to local time before stripping tzinfo:

Instead of:

dt.fromisoformat(timestamp).replace(tzinfo=None)

Use:

dt.fromisoformat(timestamp).astimezone().replace(tzinfo=None)

.astimezone() with no argument converts to the system's local timezone. For Borg 1.x (naive input), fromisoformat returns naive datetime and .astimezone()
assumes local time — so it remains a no-op. For Borg 2.x (UTC input), it properly converts UTC → local before storing.

Apply in both create.py:25 and list_repo.py:57.

Alternatives

1. Store as UTC and convert to local time only at display time (more correct but requires changing all display code)
2. Store timezone-aware datetimes by switching from SQLite DateTimeField to storing ISO 8601 strings with offset (larger refactor)

Additional context

• Low priority — only affects Borg 2.x which is still in beta
• Two-line fix for the recommended solution
• Broader datetime inconsistency across models (datetime.utcnow vs datetime.now) is a separate concern (see date/time findings #185)

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