fix(slack): one select instead of four rating buttons

Slack's mobile client gives every button in an actions block its own
full-width row, so the four ratings rendered as four stacked bars and the
nudge filled the screen (user, with a screenshot). A select is one row
and still one decision.

The interactive route now resolves both shapes — a button carries
`value`, a select carries it on `selected_option`. Reading only the
former would have left sharing working while rating silently did
nothing.

src/lib/slack-blocks.ts

@@ -69,16 +69,25 @@ export function nudgeBlocks(s: NudgeState) {
         },
       ],
     },
+    // A select, not four buttons. Slack's mobile client gives every button in
+    // an actions block its own full-width row, so four ratings became four
+    // stacked bars and the card filled the screen. One select is one row and
+    // still one decision; the extra tap to open it is the price of a nudge you
+    // can read at a glance.
     {
       type: "actions",
       block_id: "rate",
-      elements: RATINGS.map((r) => ({
-        type: "button",
-        action_id: `rate_${r}`,
-        text: { type: "plain_text", text: RATING_LABEL[r] },
-        style: r === "never" ? "danger" : undefined,
-        value: `${s.transactionId}:rate:${r}`,
-      })),
+      elements: [
+        {
+          type: "static_select",
+          action_id: "rate",
+          placeholder: { type: "plain_text", text: "How was it?" },
+          options: RATINGS.map((r) => ({
+            text: { type: "plain_text", text: RATING_LABEL[r] },
+            value: `${s.transactionId}:rate:${r}`,
+          })),
+        },
+      ],
     },
     {
       type: "context",