Add a Fit action to size columns to their contents

The datagrid already measures content -- draw() calls
regular_table.resetAutoSize() -- but only when _reset_column_size is set,
and it puts any pinned widths back immediately afterwards:

    const old_sizes = save_column_size_overrides.call(this);
    ... if (this._reset_column_size) { resetAutoSize() }
    restore_column_size_overrides.call(this, old_sizes);

So a width pinned by dragging a column edge, or carried in a saved layout,
outlives every measurement -- including the automatic ones the datagrid does
when split_by or columns change. Clearing the overrides is what releases
them; the flag then makes the next draw measure instead of reuse.

The flag is set after restore(), not before: building the model recomputes
it from what changed in the config, and would clear it again.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/src/views/Forecast.jsx

@@ -948,6 +948,48 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
     }
   }
 
+  // Size every column to its contents.
+  //
+  // The datagrid already does this -- draw() calls regular_table.resetAutoSize()
+  // -- but only when _reset_column_size is set, and it restores any pinned widths
+  // immediately afterwards:
+  //
+  //     const old_sizes = save_column_size_overrides.call(this);
+  //     ... if (this._reset_column_size) { resetAutoSize() }
+  //     restore_column_size_overrides.call(this, old_sizes);
+  //
+  // So a width pinned by dragging, or carried in a saved layout, wins over the
+  // measurement for good. Clearing the overrides is what actually releases them;
+  // the flag then makes the next draw measure rather than reuse.
+  //
+  // Order matters: the flag is set *after* restore(), because building the model
+  // recomputes it from what changed in the config and would clear it again.
+  async function fitColumns() {
+    const viewer = viewerRef.current
+    if (!viewer) return
+    try {
+      const { table: _t, ...cfg } = await viewer.save()
+      const pc = { ...(cfg.plugin_config || {}) }
+      if (pc.columns) {
+        pc.columns = Object.fromEntries(
+          Object.entries(pc.columns).map(([col, v]) => {
+            const { column_size_override, ...rest } = v || {}
+            return [col, rest]
+          })
+        )
+      }
+      await viewer.restore({ ...cfg, plugin_config: pc })
+
+      const plugin = await viewer.getPlugin()
+      if (plugin) plugin._reset_column_size = true
+      const view = await viewer.getView()
+      await plugin?.draw?.(view)
+    } catch (err) {
+      console.error('[fitColumns]', err)
+      flash(err.message || String(err), 'error')
+    }
+  }
+
   // In rollup mode the datagrid emits a grand-total column group as well as the
   // subtotals. The subtotals are the point -- they are what per-branch collapse
   // needs -- but the grand total sums across the split, and when the split is
@@ -1517,6 +1559,15 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
 
         <div className="w-px h-4 bg-gray-200 shrink-0" />
 
+        <button onClick={fitColumns}
+          title="Size every column to its contents, releasing any widths pinned by dragging or carried in a saved layout"
+          className="border border-gray-200 rounded px-1.5 py-0.5 text-gray-500 hover:border-gray-400
+                     transition-colors whitespace-nowrap">
+          Fit
+        </button>
+
+        <div className="w-px h-4 bg-gray-200 shrink-0" />
+
         {/* Expand group */}
         <div className="flex items-center gap-1.5">
           <span className="text-gray-400 uppercase tracking-wide" style={{fontSize:'10px'}}>Expand</span>