fix: handle URL-encoded @me in /users/:user_id route

[xqliu]

Summary

• /users/%40me returns 404 because %40me (URL-encoded @me) is routed to /users/:user_id instead of the literal /users/@me route
• Express automatically decodes %40 → @, so req.params.user_id becomes "@me", but the handler passes it directly to User.getPublicUser() which fails
• Add the same @me → req.user_id substitution that other :user_id routes already have (e.g. profile.ts, voice-states)

Fixes #1525

Test plan

• GET /users/%40me returns the authenticated user (same as GET /users/@me)
• GET /users/<snowflake> still works as before
• Existing /users/@me/ literal route still works

[TheArcaneBrony]

Is this AI-generated...?
This route is implemented as /users/@me explicitly rather than /users/:id (different user projections)

[oh64]

[TheArcaneBrony]

Honestly unsure how this was missed when the users directory only contains 2 directories.

[TheArcaneBrony]

Oh wait, i see the logic now behind picking the "wrong" directory now... but i'm also not sure if this is the right fix, given this would probably affect hundreds of other endpoints.
On the other hand, I also havent seen this issue happen in practice.

[oh64]

Discord treats %40me as an invalid ID and returns 404
Actually, Discord treats it normally