Accept plaintext SS userinfo emitted by Sub-Store

[enximi]

Problem

explodeSS() currently assumes that the userinfo before @ is always
URL-safe base64 when parsing links in the ss://...@server:port form.

Accepted today:

ss://<base64(method:password)>@server:port#remark

Example shape:

ss://YWVzLTI1Ni1nY206YzJWamNtVjA@server.example.com:443#Node

But Sub-Store can also emit a different shape where the userinfo is
plaintext and URL-encoded instead of base64-encoded:

ss://<method>:<urlencoded password>@server:port#remark

Example shape:

ss://aes-256-gcm:c2VjcmV0JTNEJTNE@server.example.com:443#Node

Because the parser only tries urlSafeBase64Decode(secret) in this code
path, links in the plaintext-userinfo shape are rejected.

Change

Keep the existing base64 parsing path and add a fallback that parses
URL-decoded plaintext method:password from the SS userinfo.

This preserves current behavior for existing inputs while accepting the
Sub-Store output shape above.

Verification

• Existing base64-userinfo SS links still work
• Sub-Store-generated plaintext-userinfo SS links now work
• Existing non-SS conversion behavior is unchanged

[enximi]

This PR fixes the same parsing gap as #50.

After comparing the two changes, #50 appears to cover the same root issue more completely, so I am closing this PR to avoid duplicate review and maintenance work.

Leaving this note here for traceability because this branch was produced from a real Sub-Store compatibility case on my side.

[enximi]

Closing as superseded by #50.