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

tickets/034-params-declaration-ui.md

@@ -1,41 +0,0 @@
-# 034 — Params declaration UI
-
-## Summary
-
-`POST /api/params` (ticket 004) already lets a param be declared — name,
-`message_match_regex`, `capture_group`, `kind` — and `GET /api/params`
-(`fetchParams`, `webapp/src/lib/api.ts:245`) already feeds LogPreview's
-"Columns" picker (`webapp/src/components/LogPreview.tsx:63`) so a declared
-param can be shown as a log table column. But nothing in the webapp can
-*create* one — the only way to declare a param today is a raw `POST
-/api/params` call. Add a UI for it, following the same declare-then-list
-pattern `Metrics` already uses for `POST /api/metrics`
-(`webapp/src/components/Metrics.tsx`).
-
-## Scope
-
-- A "Params" section in the primary UI (alongside Log viewer / Metrics /
-  Dashboards in `App.tsx`), listing currently-declared params (name, regex,
-  capture group, kind) via `fetchParams`.
-- A declare form: name, `message_match_regex`, `capture_group`, kind
-  (string/number/timestamp — mirror `MetricPrimitiveKind`), submitting via a
-  new `declareParam` call to `POST /api/params`.
-- Surface `400` validation errors from the backend (bad regex, unknown
-  capture group) inline on the form, matching how `Metrics`' declare form
-  reports its own validation errors.
-- After a successful declare, refresh the params list so it (and
-  LogPreview's Columns picker) immediately reflects the new param without a
-  page reload.
-- Contextual "?" help icon on the section, matching [021](021-docs-contextual-help.md)'s pattern.
-
-## Acceptance criteria
-
-- A param can be declared from the UI and immediately appears in LogPreview's
-  Columns picker without a page reload.
-- An invalid regex or unknown capture group shows the backend's error message
-  on the form instead of failing silently.
-- The declared-params list matches `GET /api/params` after a page reload.
-
-## Depends on
-
-- [021 — Docs contextual help](021-docs-contextual-help.md)