Show the trendline Side control only when it can do anything

Snapping now always lands an anchor on a bar extreme, and which extreme it is
decides the side — a high is resistance, a low is support. That left the Side
dropdown unable to affect the result: it was overridden on every drawn line.

It now appears only when "Snap to highs/lows" is off, the one case where there
is no extreme to infer from. With snapping on the row reads "Side auto" instead,
so the behaviour is stated rather than implied by a control that does nothing.
An "Auto" option in the dropdown would have been the same no-op wearing a label.

Verified in a browser both ways, and that the inference itself holds with the
dropdown left on its default: drawing above the candles yields resistance
snapped to the bar high, drawing below yields support snapped to the bar low.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/IMPLEMENTATION_PLAN.md

@@ -1489,3 +1489,13 @@ resolving to the 08:30 bar on 15m and the 08:00 bar on 1h, never edge-parked,
 and returning to its original x on the way back. Edge-parking still works where
 it should — a comment scrolled 400 bars out parks right and comes back on
 return to live.
+
+**The trendline Side control became inert.** Once snapping always lands on a
+bar extreme, the side is inferred from *which* extreme — a high is resistance, a
+low is support — so the dropdown could no longer affect anything. It now appears
+only when "Snap to highs/lows" is off, which is the one case where there is no
+extreme to infer from; otherwise the row reads "Side auto". Verified both ways:
+snap on shows the note and no dropdown, snap off shows the dropdown.
+
+`created_at` (epoch seconds) is already stored on every drawing and returned by
+`GET /api/drawings`, so filtering by age needs UI only, not a migration.

static/index.html

@@ -65,7 +65,10 @@
               <label>Width<select v-model.number="drawWidth" aria-label="Trendline width"><option v-for="width in [1,2,3,4]" :value="width">{{ width }}px</option></select></label>
             </div>
             <div class="row">
-              <label>Side<select v-model="drawSide" aria-label="Trendline side"><option value="support">Support</option><option value="resistance">Resistance</option></select></label>
+              <!-- Side is inferred from the extreme you snap to, so it only has
+                   anything to say when snapping is off. -->
+              <label v-if="!snap">Side<select v-model="drawSide" aria-label="Trendline side"><option value="support">Support</option><option value="resistance">Resistance</option></select></label>
+              <span v-else class="side-auto" title="A high is resistance, a low is support">Side <b>auto</b></span>
               <label class="check"><input type="checkbox" v-model="snap">Snap to highs/lows</label>
             </div>
           </div>

static/style.css

@@ -76,3 +76,4 @@ aside { padding:16px; }h2 { margin:0 0 12px; color:var(--muted); font-size:11px;
   border-radius:4px; background:transparent; color:var(--muted); cursor:pointer; }
 .tool-body textarea { width:100%; min-width:0; font:inherit; font-size:11px; padding:4px 6px; resize:vertical;
   border:1px solid var(--line); border-radius:5px; background:transparent; color:var(--fg); }
+.side-auto { align-self:end; padding-bottom:6px; font-size:10px; color:var(--muted); }.side-auto b { color:var(--fg); font-weight:600; }