Remove path gateway ipfs.io, use dweb.link everywhere

[lidel]

Current state

We have:

• "Default Public Gateway" set to ipfs.io
• "Default Public Subdomain Gateway" set to dweb.link

Problem

We've introduced subdomain gateways to solve multiple issues in browser context, and are moving all web use cases towards that model. Soon, we will start locking down path gateways (ipfs/in-web-browsers#157) which means website/dapp links copied by users will stop working correctly if loaded via ipfs.io.

IPFS Companion should remove the footgun of path gateway by removing it from GUI.

Desired state

• There should only be "Default Public Gateway" set to dweb.link.

Side-quest: migrate existing user settings

We need to write a migration that reads "Default Public Subdomain Gateway" and uses that (if it is other than dweb.link)

cc @darobin @autonome for sanity check
@whizzzkid @SgtPooki fysa this impacts end-user security, and should be prioritized

[lidel]

Addressed in #1371. Copy Shareable Link now prefers the subdomain gateway when one is set, falling back to the path gateway only when the CID is too long for a DNS label. If no public gateway is configured, it copies the native ipfs:// or ipns:// address instead of routing through a third party.

That removes the path-gateway footgun without dropping path support outright, since some CIDs still need it. Closing this; open a new issue if you want to flip the defaults to native-by-default.