v0.15.0

Admin restructure, cross-device key sync, and a critical chat fix. Driven
by live mobile testing.

Fixed

• All chat requests were 400-ing. The configure_agent tool's
  function-declaration schema used an array-valued (union) type
  (["string","null"]), which Gemini rejects with a 400 — and since the
  tool ships on every request, it broke every chat turn. Switched to single
  types. Added cargo test builtin_tool_schemas_have_no_union_types, a
  network-free lint over every builtin tool schema so this can't recur.
• DEFAULT_MODEL → gemini-3.5-flash. gemini-2.5-flash now 400s;
  3.5-flash is the current model (verified against the live API).
• Tool-call blocks now auto-scroll the transcript like text does (they
  previously appended below the fold).
• APP_VERSION auto-tracks the crate version (env!("CARGO_PKG_VERSION")),
  so the admin footer can't drift from the release.

Changed

• Admin is now a full-page tabbed panel — Agent (prompt · tools ·
  publish) / Account (identity · API key · linked devices · security) /
  Usage (registered-subdomain count · session token total). The agent card
  was folded in from the old right rail (rail + mobile "agents" tab
  removed).
• Feedback is write-only — the public "recent" list is gone (it exposed
  everyone's submissions on a permanent on-chain log); submit still goes
  on-chain. Apex agents list shows main/alt labels instead of an "act"
  button; the apex shows "loading agents…" while the list resolves; the
  boot "loading" text is centered.

Added

• On-chain encrypted API-key sync (cross-device). "Sync to seed" seals
  the Gemini key with a wallet-seed-derived key (via the apex signer) and
  stores the ciphertext on-chain (sponsored setMetadata); "restore" pulls
  • decrypts it on any device that has imported the seed — no re-paste.
    Note (accepted testnet tradeoff): the decrypt op is honored for any
    localharness origin; Gemini keys are free/revocable.