fix: offline advisory DB unavailable error should include the sync command to create it

[sonukapoor]

Problem

When --offline or --offline-db is used but the database does not exist or cannot be opened, the error is:

Error: Offline advisory database is not available: <reason>

There is no hint on how to create the database. Users — especially those setting up offline mode for the first time — are left stuck.

Expected behaviour

Error: Offline advisory database is not available: <reason>
To build it, run: cve-lite advisories sync
Or to save it to a custom path: cve-lite advisories sync --output /path/to/advisories.db

Where to look

src/index.ts — around line 153 where the offline DB availability check throws.

Acceptance criteria

• The error message includes the cve-lite advisories sync command
• When --offline-db <path> was passed, the hint also shows --output <path> so the user knows where to save it
• Tests updated to assert the new message text

[dagangtj]

PR submitted: #405

Offline advisory database unavailable error now includes the sync command:

• Without --offline-db: 'To build it, run: cve-lite advisories sync'
• With --offline-db: 'To build it, run: cve-lite advisories sync --output '

Ready for review!