fix(web): only write settings on a real header edit; label rows by position (#1915)

Round-3 review follow-ups on #2038.

`buildHeaderSettingsPatch` treated the mere presence of stored headers as a
change, so saving an id or URL edit re-sent the modal's settings snapshot —
taken when it opened — and could overwrite a metadata or OAuth change made
in the settings form since. It now compares the submitted headers against
the stored ones and omits `settings` when they match, which is what makes
the PUT route preserve the node. Order counts as a difference: it is what
the form round-trips and what the user sees.

Row labels carried the key alone, so two rows sharing a key — mid-edit, or a
duplicate a server persisted — gave both rows' controls identical accessible
names, the thing the labelling exists to prevent. The row number is now
always part of the label rather than a blank-key fallback.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/components/elements/KeyValueRows/KeyValueRows.stories.tsx

@@ -27,18 +27,18 @@ export const Populated: Story = {
   play: async ({ args, canvasElement }) => {
     const canvas = within(canvasElement);
     const value = await canvas.findByRole("textbox", {
-      name: "header value, Cookie",
+      name: "header value, Cookie, row 1",
     });
     await expect(value).toHaveValue("branch=feature-x");
 
     await userEvent.click(
-      canvas.getByRole("button", { name: "Remove header, X-Env" }),
+      canvas.getByRole("button", { name: "Remove header, X-Env, row 2" }),
     );
     await expect(args.onRemove).toHaveBeenCalledWith(1);
   },
 };
 
-// A row whose key is still blank falls back to a positional name rather than
+// A row whose key is still blank is named by position alone rather than
 // announcing nothing.
 export const BlankKeyRow: Story = {
   args: { items: [{ key: "", value: "" }] },