fix(identities): send unlink provider as a query param

[Adron]

Summary

Closes #77. Settings -> Connected accounts -> Disconnect always failed with 400 {"error":"provider is required"}.

APIClient.unlinkIdentity sent the provider in a camelCase JSON body via deleteCamel, but DELETE /api/user/identities reads it from the query string (searchParams.get('provider')) and never parses a body, so the value was always null. The call now uses the bodyless delete(_:) against /api/user/identities?provider=<encoded>.

providerId is gone from the signature: the route never reads it and deletes by { userId, provider } via deleteMany, so it removes every identity row stored under that provider value. That is recorded in a doc comment on the method rather than worked around here. In practice Mastodon rows are stored per instance (mastodon:techhub.social), so distinct instances are unaffected; the existing confirmation copy ("You'll no longer be able to sign in with X") is already provider-scoped.

What's included

• unlinkIdentity(provider:) sends the provider as a query parameter and an empty body.
• Percent-encoding uses .urlQueryAllowed minus +&=?#/. That set permits the sub-delimiters, and the backend reads params through URLSearchParams, which decodes a literal + as a space.
• Call site in LinkedIdentitiesView updated; its 401 path still goes through authState.handleUnauthorized(), unchanged.
• deleteCamel's doc comment no longer lists identity unlink as a body-scoped route.
• Nine unlinkIdentity tests in APIClientIdentitiesTests.

No new files, so no project.pbxproj change. Out of scope by design: no token-health / "check connection" work (that is #92), and verifyIdentity is untouched.

Testing

• Full suite, serialized, E2E skipped: 1153 tests, 0 failures (-destination id=D9B21F4D-933E-4871-98AE-B3E65427BCF5, dedicated -derivedDataPath, -parallel-testing-enabled NO, -skip-testing:InterlinedListTests/E2EReadOnlyTests). No compiler warnings.
• Regression check: with the production method reverted to the body-based call, the four new assertions fail (sendsProviderAsQueryParameter, sendsEmptyBody, mastodonProvider_keepsInstanceSuffix, providerWithQueryDelimiters_percentEncodesThem), so the tests bite.
• New coverage: query parameter present, body is nil, verb DELETE + path /api/user/identities, mastodon:instance survives encoding, + & = are percent-encoded and round-trip through URLComponents, plus the 401 / 403 / 404 error paths (404 surfaces the route's Identity not found).
• Not verified: the live disconnect against the real API — that needs a signed-in account with a linked identity. Confirm by disconnecting an account in Settings and refreshing.

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