fix(connections): URL import rejects unescaped passwords without a hint, mangles Windows SQLite paths, and unbrackets IPv6 #1315

Problem

lib/features/connections/connection_url_parser.dart:

1. Unescaped specials. postgres://user:p@ss@host/db, ...:p#ss@... and mysql://root:pa/ss@host/db make Uri.tryParse return null, so the user gets only Invalid URL/URI format. (checked with dart run). The cause is common (a raw password from a cloud console) and the message says nothing about percent-encoding.
2. Windows SQLite. sqlite:///C:/data/app.db gives the path /C:/data/app.db (uri.path), which does not exist on Windows.
3. IPv6. uri.host is ::1 without brackets; building a URI from the host later (Mongo buildConnectionUri, the default name) yields an invalid string.

Scope

• When parsing fails and the text has an @ after a : in the authority, retry with the userinfo percent-encoded, or say special characters in the password must be percent-encoded (@ is %40).
• Strip the leading slash of a /X:/... drive path on Windows.
• Re-bracket IPv6 hosts where a URI is built.
• Tests for each.

Acceptance

• A URL with a raw @ or # in the password imports, or fails with the encoding hint.
• sqlite:///C:/x.db opens C:/x.db on Windows.
• An IPv6 host round-trips.