Add Params declaration UI (ticket 034)

New Params panel (Log viewer / Params / Metrics / Dashboards) declares a
bare param via POST /api/params, mirroring Metrics' declare-then-list
pattern including inline 400 error surfacing. Lifted the params list from
LogPreview's own effect into App.tsx so declaring a param there updates
the Columns picker immediately, without a page reload.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/components/Docs.tsx

@@ -116,9 +116,9 @@ const SECTIONS: DocSection[] = [
           the log table and as a comparable value in the filter bar.
         </p>
         <p>
-          Declaring a param independent of a metric is supported by the API (<Code>POST /api/params</Code>), used
-          today via the Metrics panel's declaration form and read back by the log preview's column picker; there
-          isn't yet a standalone "declare a bare param" form in the UI.
+          The Params panel declares a bare param directly (<Code>POST /api/params</Code>) — the same request the
+          Metrics panel's declaration form makes per param matcher. Declaring one there immediately updates the log
+          preview's column picker, with no page reload needed.
         </p>
       </div>
     ),