Add selectable declared-param columns to LogPreview (ticket 015)

Extend GET /api/logs to derive each declared param's value per log line
(regex + capture group + kind, matching that line's text) and include it as
a params map on LogEntryDto, omitting entries that don't match rather than
sending nulls. Add GET /api/params (list_params, mirroring list_metrics) so
the frontend can discover currently-declared param names. In LogPreview, add
a column-picker popover backed by that list, with the selection persisted in
localStorage under `ottyl.logPreview.visibleParamColumns`.

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

webapp/src/lib/api.ts

@@ -1,7 +1,33 @@
+/// `MetricPrimitive` is a Rust enum with derived `Serialize`, so its JSON
+/// form is externally tagged, e.g. `{ "String": "info" }`, `{ "Number":
+/// { "Int": 5 } }`, `{ "Timestamp": 1710000000000 }`.
+export type MetricPrimitive =
+  | { String: string }
+  | { Number: { Int: number } | { Float: number } }
+  | { Timestamp: number }
+
+export function metricPrimitiveToDisplayString(value: MetricPrimitive): string {
+  if ('String' in value) return value.String
+  if ('Timestamp' in value) return String(value.Timestamp)
+  if ('Int' in value.Number) return String(value.Number.Int)
+  return String(value.Number.Float)
+}
+
 export interface LogEntry {
   sequence: number
   timestamp_ms: number
   text: string
+  /// Derived param values (declared via `POST /api/params`) that matched
+  /// this line's text, keyed by param name. A param with no match on this
+  /// line is simply absent from the map.
+  params: Record<string, MetricPrimitive>
+}
+
+export interface ParamDeclaration {
+  name: string
+  message_match_regex: string
+  capture_group: string
+  kind: 'string' | 'number' | 'timestamp'
 }
 
 export interface TimelineBucket {
@@ -54,3 +80,7 @@ export function fetchTimeline(params: {
 export function fetchStats(): Promise<Stats> {
   return getJson('/api/stats')
 }
+
+export function fetchParamDeclarations(): Promise<ParamDeclaration[]> {
+  return getJson('/api/params')
+}