Providers as data; add Ollama and Custom endpoints

Every gateway provider is now an entry in providers.json - endpoint and auth
template, how its model list is read, how it is probed before a switch, what
setup asks, which doctor checks apply, and its title, colour and logo -
installed next to the presets and read by all three consumers: the bash CLI
(through cm-json.py), the Windows script, and the bar widget (through
health.json). anthropic stays built in; it is the native login, not a gateway.

Behaviour that differs in kind stays in code, chosen by name from the entry:
catalogue parsers (openrouter, lmstudio, ollama, openai, static), probe rules
(always, lenient, local), and named doctor checks. A provider that reuses them
is an entry and a default preset, with no code. The widget draws providers
from health.json, so a new one needs no QML change and no shell restart.

The existing three are unchanged in behaviour: their blank presets come out
byte-identical from the file, and setup, doctor, models and the picker run the
same checks through the generic paths.

Ollama: local server on :11434, placeholder token, one model for every tier,
models from /api/tags. doctor reads the context each loaded model actually runs
with (/api/ps) and its maximum (/api/show), because Ollama defaults to 4096
tokens unless OLLAMA_CONTEXT_LENGTH is set and silently truncates past it. A
bare model name matches its :latest tag.

Custom: any Anthropic-compatible endpoint. Ships with no address and is refused
until it has one; key optional; models from /v1/models when the endpoint has a
list, and a lenient probe so a proxy without one is not blocked.

Also: preflight (and Set-ClaudeMode on Windows) refuses a preset with no
server address; the server form, setup and set-auth use each provider's own
default URL, key name and placeholder token instead of LM Studio's; the
Windows build gains the no-models and no-address guards it never had.
Tested on Linux against fake Ollama/Custom servers, and on Windows 5.1 in a
USERPROFILE sandbox on winbox.

1.12.0.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

presets/ollama.json

@@ -0,0 +1,22 @@
+{
+  "provider": "ollama",
+  "description": "Local Ollama server. Setup picks the model from whatever the server has pulled.",
+  "baseUrl": "http://127.0.0.1:11434",
+  "auth": {
+    "mode": "literal",
+    "token": "ollama"
+  },
+  "models": {
+    "opus": "qwen3-coder",
+    "sonnet": "qwen3-coder",
+    "haiku": "qwen3-coder",
+    "fable": "qwen3-coder"
+  },
+  "subagentModel": "inherit",
+  "gatewayModelDiscovery": false,
+  "extraEnv": {
+    "CLAUDE_CODE_ATTRIBUTION_HEADER": "0"
+  },
+  "contextTokens": 65536,
+  "configured": false
+}