[efiten]

Closes #751

Stacked on #750 (base fix/companion-link-no-reassign). Merge #750 first, then retarget this PR to master.

Register and link-companion now need proof that the caller holds the companion, not only its public pubkey.

• POST /api/auth/companion-challenge {companion_pubkey} returns a challenge. It is stateless: expiry, a 16-byte nonce and an HMAC-SHA256 over pubkey, expiry and nonce. The key is random per process. The challenge is bound to the pubkey and valid for 5 minutes, and a restart voids open challenges.
• Register and link-companion take challenge and signature, an Ed25519 signature over core-hunter-link: + challenge. Without a valid proof they answer 403 companion_proof_invalid. In register the proof check runs before the companion-taken check, so a caller without the key cannot learn whether a pubkey is linked.
• The app gets the challenge and has the connected radio sign it through createSigner, the same CMD_SIGN path the broker tokens use. Firmware CMD_SIGN_FINISH signs with self_id and asks for no confirmation on the radio (examples/companion_radio/MyMesh.cpp:1752). With no radio connected, the app shows a message and sends no request.

Tests:

• internal/auth/companionproof_test.go: valid proof accepted; refused for another key, a challenge for another pubkey, an expired challenge, another server's key, a tampered challenge, a signature without the prefix, non-hex input and a short pubkey. Mutation-checked: dropping the HMAC, expiry or prefix check each turns a case red.
• internal/httpapi/auth_test.go: TestCompanionLinkRequiresProof (no proof, or a forged one, gives 403 on both endpoints, and register leaves no account behind) and TestCompanionChallenge. Existing register/link tests now send a real proof. Disabling either handler check turns the test red.
• app/src/__tests__/auth.test.js: companionProof signs the prefixed challenge, and returns null without a challenge or without a signature. The body builders carry the proof.

go vet ./... and go test ./... pass in server/. npx vitest run passes in app/ (1748), and vite build succeeds.

Not done:

• Not tested on a real radio. Only dummy Ed25519 keys were used, and whether the firmware signature verifies under Go's ed25519.Verify was not run end to end. The firmware uses the standard ref10 ed25519_sign (lib/ed25519/sign.c), and the broker tokens already depend on the same signatures.
• An older cached app version cannot register or link until it updates, because the server now requires the proof.
• Companions that are already linked are not re-checked.