feat: default to average downsampling

Average consolidation makes every plotted point the true arithmetic mean of
its interval, which is what the Ganglia-style plot appearance depends on.
LTTB deliberately keeps extremes instead, so a plot downsampled with it
cannot be read as interval means.

resolveResampleAlgo used to return an empty string when neither the user nor
the config had chosen an algorithm, and cc-lib's GetResampler maps that to
LTTB. Route every fallback through config.ResampleAlgo() and ship "average"
as the generated and example default.

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

web/frontend/src/config/user/PlotRenderOptions.svelte

@@ -301,7 +301,10 @@
             </div>
           {/each}
           <div id="resampleAlgoHelp" class="form-text">
-            Algorithm used when downsampling time-series data. LTTB preserves visual shape, Average smooths data, Simple picks every Nth point.
+            Algorithm used when downsampling time-series data. Average (the
+            default) makes every plotted point the true mean of its interval,
+            LTTB preserves visual shape by keeping extremes, Simple picks every
+            Nth point.
           </div>
         </div>
         <Button color="primary" type="submit">Submit</Button>