feat(rust): mod configuration from the site, and an editor that will not rewrite a float

R18's two tiers: a form generated from a config file's own values, and raw JSON
for what a form cannot express. Admin → Rust mod config, one live round trip per
action, nothing cached between a browser and a game host's disk.

`configEdit.js` is the part that could not be done naively. JavaScript cannot
tell `1` from `1.0`, and both mod frameworks deserialize a config into typed C#
classes — so a read-modify-write silently rewrites every whole-numbered float as
an integer on fields nobody touched, and a plugin that then throws at load does
not come back. It never parses, mutates and re-serialises: it records the SOURCE
SPAN of every value and splices literals into them, so an untouched `1.0` is
still `1.0` and a number an admin types travels as text the whole way (D35/D36).

The bridge's own config is editable with `Host`, `Port` and `ServerId` locked,
in the form and in the raw tier, because either would cut the link carrying the
edit or strand every row this site holds (D38). Credentials render masked with a
reveal; the raw tier shows them (D37) and the audit trail never does.

`rust_config_writes` records every save including the refused and the rolled
back — an operator asking why a setting is not what they set needs to see that
somebody tried.

Three defects a browser walk found that 179 green tests did not:

* every save of the bridge's own config was refused while the page said the
  opposite — a `<select>` whose value matches no `<option>` shows the first one,
  so the reload guess `RunicGateway` was on the wire and "nothing" was on the
  screen;
* `btn ghost` is not a class this platform defines (`.btn-ghost` is), so every
  secondary button in this module has rendered as a primary one since phase 7 —
  here it made the open file and the active tier indistinguishable;
* a save's refusal rendered at the top of a long form, far from the button.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PMH6bw1jXMgbyF3ZWGEzSM

client/src/routes/admin/UserRustSections.jsx

@@ -83,7 +83,7 @@ function LinkPanel({ userId, link, onRemoved }) {
             </div>
           )}
         </div>
-        <button type="button" className="btn ghost" onClick={unlink} disabled={busy} style={{ flex: 'none' }}>
+        <button type="button" className="btn btn-ghost" onClick={unlink} disabled={busy} style={{ flex: 'none' }}>
           {busy ? 'Unlinking…' : 'Unlink'}
         </button>
       </div>
@@ -180,7 +180,7 @@ function PermissionsPanel({ userId, data, onChanged }) {
           </span>
           <button
             type="button"
-            className="btn ghost"
+            className="btn btn-ghost"
             disabled={busy}
             onClick={() => act(() => api.adminUserPermissions.revoke(userId, row.id))}
             style={{ flex: 'none' }}