fix(android): list and apply chat models without a Desktop Gateway

[software-greg]

The per-chat model picker is coupled to the Desktop Gateway WebSocket: _showModelSelector returns early unless a gateway client exists, and applying a choice pushes it over that socket via setSessionModel / setSessionReasoning. Any deployment without a reachable gateway therefore has a permanently dead picker, even though chat, sessions and history all work.

Listing needs only api/model/info and api/model/options over dashboard REST, and the API server already resolves a per-request model field, so the socket is not reqd for either half of the feature.

• list models through DashboardClient when no gateway is configured;
• keep the selection local and persisted instead of pushing it over the socket;
• carry the chosen model as the model field on the chat request, which sendMessageStreaming already accepts.

Reasoning effort remains gateway-only and is simply not applied in this mode.

Gateway turn recovery and its journal invariants are untouched: with a gateway configured every path behaves as before.

Motivation: we run Hermes headless and reverse-proxy only a handful of read-only dashboard GETs to the android client -- api/model/{info,options} for this picker, plus api/{memory,skills,cron/jobs} for the read-only tabs -- while deliberately not routing api/config, which returns provider keys and passwords. Every one of those tabs already works over REST; the model picker was the only feature that reached for the gateway socket, and only to read two endpoints we already publish.

[software-greg]

Related issues:

• Remote Gateway transport never activates against stock Hermes (turn_recovery capability doesn't exist upstream) + legacy fallback lacks resume re-sync #84 - gateway transport never activates against stock Hermes, so the picker is dead there too. This fixes that case.
• API and Port 8642 Requirement #90 - partially: removes the picker's dependency on the gateway, but doesn't otherwise address the 8642 requirement.
• fix: remove hardcoded desktop gateway URL default and allow clearing optional fields #86 - complementary, no overlap. It touches lib/main.dart only; fix: remove hardcoded desktop gateway URL default and allow clearing optional fields #86 makes the empty-gateway state reachable, this makes the picker work in it.

[rusty4444]

Hermes automation review — APPROVED (not auto-merging this pass)

• Well-scoped, conservative fix: _modelListingClient() only builds a DashboardClient when no Desktop Gateway is configured; the restClient! null-safety is sound by construction.
• Verified against the repo: constructor args match DashboardClient, referenced SavedConnection getters exist, and gateway-connected sessions still route through the existing gateway message paths — no regression to the desktop-gateway flow.
• ⚠️ This branch has no CI checks reported — merge only once CI is visible and green.
• ⚠️ Merge-order: #95/#96 also modify chat_screen.dart; expect a rebase after those land.