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

src/dto.rs

@@ -9,6 +9,13 @@ pub struct LogEntryDto {
     pub sequence: u64,
     pub timestamp_ms: i64,
     pub text: String,
+    /// Derived param values (ticket 001's `ParamDeclaration`s) for this line,
+    /// keyed by param name. Only params whose regex actually matched this
+    /// line's text (and whose captured group parsed as the declared `kind`)
+    /// are present — a non-matching param is omitted rather than sent as
+    /// null, so the frontend can render a blank/dash for anything missing
+    /// from the map.
+    pub params: BTreeMap<String, MetricPrimitive>,
 }
 
 #[derive(Debug, Clone, Serialize)]