fix: what the third round broke, and the first batch's review

MCP preview after the server lost a session (it expired, or the MCP
process restarted):
- Every server state has an id, made when the state is created. The tab
  notices a new id even when the version numbers happen to match, and
  every push names the state it was based on, so one based on a lost state
  is refused, also when it comes before the tab's first poll (the server
  recovers the saved file first).
- The tab keeps the newest canvas XML, saved or not. When the server knows
  nothing (no file) or exactly what the tab last saved, the canvas wins and
  is saved, so edits made while the server was down are kept. Otherwise
  the server's diagram (an AI write the tab missed, a cleared document
  that was saved) is shown and the tab's copy goes to History.
- Late answers to an old state's push or poll are dropped; a failed push
  says the server is unreachable; Download as .drawio saves the canvas.

Settings and server:
- Saved providers this version does not know stay in storage with their
  keys, and sending no longer trips over them.
- The desktop "Ollama (Local)" preset with a key goes to local Ollama
  again; a server model's Ollama URL variable is read; the admin panel
  writes Ollama Cloud's URL for a key without one.
- Provider error texts show again in the desktop app and for EdgeOne.
- .env: a quoted value followed by a comment ending in a quote is read as
  dotenv reads it; unquoted values are unchanged.
- Desktop app: the next launch opens the port where a chat was last
  saved; a launch elsewhere that saves nothing does not move it, and a
  page with no chats lets the next launch try the other port once.
- The Test button no longer stays busy after another tab changed the key.
- A completed append_diagram is no longer undone by an earlier failed
  edit's preview; a file read once in vain is saved again once it is read
  or gone.

From the first batch's review:
- The admin panel's Test of an entry without a URL now tests the server's
  <P>_BASE_URL, where chat sends the entry's key; chat is unchanged (the
  first fix rerouted working setups).
- The model list ends downloads that are too large, accepts answers
  without a body, and keeps the "redirects are not allowed" explanation.
- A test covers the preview's History rendering.

lib/ai-providers.ts

@@ -997,17 +997,15 @@ export function getAIModel(clientOverrides?: ClientOverrides): ModelConfig {
                 ? overrides?.apiKey || undefined
                 : resolveApiKey(overrides, "OLLAMA_API_KEY")
             // Like other providers, a user's key never goes to the server's
-            // base URL. A key without a base URL is an Ollama Cloud key
-            // (local Ollama has no keys); without either, the SDK's local
-            // default.
+            // base URL: without a URL of their own it goes to Ollama Cloud.
+            // The server's key goes to OLLAMA_BASE_URL (or a server model's
+            // own variable), else to the SDK's local default: the desktop
+            // app's "Ollama (Local)" preset puts its key field there too.
             const baseURL =
                 overrides?.baseUrl ||
                 (overrides?.apiKey
                     ? PROVIDER_INFO.ollama.defaultBaseUrl
-                    : process.env.OLLAMA_BASE_URL ||
-                      (apiKey
-                          ? PROVIDER_INFO.ollama.defaultBaseUrl
-                          : undefined))
+                    : resolveBaseUrlEnv(overrides, "OLLAMA_BASE_URL"))
             model = createOllama({
                 ...(baseURL && { baseURL }),
                 ...(apiKey && {
@@ -1043,9 +1041,8 @@ export function getAIModel(clientOverrides?: ClientOverrides): ModelConfig {
                     : `${provider.toUpperCase()}_BASE_URL`
             // A local default (SGLang's 127.0.0.1) only fills the settings
             // form; the server must not call its own machine for it. With a
-            // user's key, or an admin entry's own (empty) URL variable, the
-            // OpenAI SDK would read the server's OPENAI_BASE_URL, so name
-            // the official endpoint.
+            // user's key the OpenAI SDK would read the server's
+            // OPENAI_BASE_URL, so name the official endpoint.
             const defaultUrl = PROVIDER_INFO[provider].defaultBaseUrl
             const publicDefault = defaultUrl?.startsWith("https://")
                 ? defaultUrl
@@ -1058,10 +1055,7 @@ export function getAIModel(clientOverrides?: ClientOverrides): ModelConfig {
             const baseURL =
                 configuredBaseURL ||
                 (SDK_KNOWS_ENDPOINT.has(provider) &&
-                !(
-                    provider === "openai" &&
-                    (overrides?.apiKey || overrides?.baseUrlEnv)
-                )
+                !(provider === "openai" && overrides?.apiKey)
                     ? undefined
                     : publicDefault)
             // With a user's Azure key the SDK would read the server's
@@ -1097,6 +1091,23 @@ export function getAIModel(clientOverrides?: ClientOverrides): ModelConfig {
     return { model, providerOptions, modelId, provider }
 }
 
+/**
+ * The server's <P>_BASE_URL for a provider, which getAIModel uses for a
+ * server model without a URL variable of its own (an admin panel entry
+ * without a URL). Bedrock, EdgeOne and Ollama (the panel writes
+ * OLLAMA_BASE_URL itself) have none.
+ */
+export function globalBaseUrl(provider: ProviderName): string | undefined {
+    if (["bedrock", "edgeone", "ollama"].includes(provider)) return undefined
+    const name =
+        provider === "vertexai"
+            ? "GOOGLE_VERTEX_BASE_URL"
+            : provider === "gateway"
+              ? "AI_GATEWAY_BASE_URL"
+              : `${provider.toUpperCase()}_BASE_URL`
+    return process.env[name] || undefined
+}
+
 /** The provider of the server's own config: AI_PROVIDER, or the one with a key */
 export function getServerProvider(): ProviderName | null {
     return (process.env.AI_PROVIDER as ProviderName) || detectProvider()