Public gateway URLs are now empty by default, so "Copy Shareable
Link" and friends copy native ipfs:// and ipns:// URIs. A new opt-in
toggle restores public gateway URLs for sharing with people who do
not run IPFS.

- options: empty publicGatewayUrl/publicSubdomainGatewayUrl defaults
  plus usePublicGatewaysForShare (off, disabled until a public gateway
  URL is set)
- ipfs-path: resolveToNativeUri plus toggle-aware resolveToShareableUrl
  and resolveToPermalink (Copy Shareable and Snapshot Link);
  resolveToPublicUrl falls back to native on empty gateway
- public shareable links follow the "Use Subdomains" preference: on
  uses the subdomain gateway, off the path gateway, and whichever one
  is configured when only one is set
- native URIs use case-insensitive roots since browsers treat them as
  origins: base32 CIDv1 for ipfs://, base36 libp2p-key CIDv1 for
  ipns:// peer ids (DNSLink hostnames unchanged)
- empty gateways handled across state, request, dnslink, import and
  inspector: sameGateway/dropSlash are null-safe and browser-loading
  paths fall back to the local gateway
- recovery page: with no public gateway configured it shows the native
  ipfs:// address, drops the public-gateway wording, and offers to
  install a local IPFS node
- options UI: public gateway fields moved below the local gateway;
  path gateway renamed to make its shape explicit; both documented
  with link examples and links to the path/subdomain gateway specs

Making public gateways optional left several consumers resolving to
native ipfs:// URIs they could not use, and routed offline failures to
dead ends. Degrade each consumer to a loadable URL or the recovery page.

- resolveToPublicUrl: use the subdomain gateway for content paths when
  only it is set, keep subdomain requests native rather than downgrade
  them to a path gateway (origin isolation), and repair the ipns://
  DNSLink protocol swap that URL.protocol assignment silently dropped
- recovery: send failed requests to the recovery page with the native
  URI when no loadable gateway remains, including failures at the
  configured gateway itself; only reopen on the local gateway when the
  node is online; limit HTTP recovery to 5xx
- preload: noop without a public gateway, otherwise pick a random
  configured one
- linkify: fall back to the local gateway so injected hrefs stay http(s)
- sharing: reset usePublicGatewaysForShare when both gateway URLs are
  cleared; share helpers no longer re-parse an already-decoded path
- sameGateway: match on a label boundary so nondefaultipfs.io is not
  treated as ipfs.io

Match the defaults ipfs/ipfs-webui#2482 shipped: fresh installs prefill
ipfs.io and dweb.link and copy dweb.link subdomain URLs, while clearing
the gateway URLs switches copied links to native ipfs:// and ipns://
URIs.

- options: defaults live in DEFAULT_* constants; a future flip to
  native-by-default changes those plus a migrateOptions entry
- ipfs-path: share links prefer the subdomain gateway and fall back to
  the path gateway, then native; roots longer than the 63-char DNS
  label limit no longer produce hostnames that cannot resolve
- options/store: clearing the last gateway URL no longer flips the
  share toggle off; the fallback is dynamic, so re-adding a URL
  restores public sharing
- recovery: resolve a native URI via the background once the node is
  online instead of navigating to ipfs://, which Chromium drops
- popup: show the local gateway address when no public one is set
- locales/en: describe the new behavior on the Preferences screen