docs(reset): "hold until the screen starts to flash" terminology

Renames the user-facing description of the BOOT-button factory reset
across the codebase. The threshold remains 5 s (RESET_HOLD_MS) but
"hold for 5 seconds" misled users: total wall-clock time-to-visible-
change includes ~20 s of e-ink redraw after the threshold fires, and
a too-short press now wakes the device into a normal poll cycle (a
side effect of the EXT0 wakeup we just added). "Until the screen
starts to flash" matches what the user actually sees.

  - Remove-this-frame modal gains a small aside describing the
    physical reset for the new owner, with the new terminology and
    a callout that a brief tap just refreshes the image.
  - CLAUDE.md and the operation.h comment near the EXT0 wake call
    use the same phrasing.
  - feedback_reset_terminology.md memory locks the rule for future
    edits — never write "hold for 5 seconds" in user copy.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/views/HomeView.vue

@@ -251,6 +251,12 @@
             this frame from your account and unlinks it from your photos so
             the next owner can claim it fresh. This can’t be undone.
           </p>
+          <p class="home-view__remove-confirm-aside">
+            On the frame itself, the new owner can also do a factory reset
+            by holding the small button on the back until the screen starts
+            to flash. (If they only tap it briefly, the frame just refreshes
+            its current image — keep holding until it flashes.)
+          </p>
           <div class="home-view__remove-confirm-actions">
             <button
               type="button"
@@ -1106,7 +1112,16 @@ async function saveSettings() {
     font-size: var(--text-sm);
     color: var(--color-text);
     line-height: 1.5;
+    margin-bottom: var(--space-2);
+  }
+
+  &__remove-confirm-aside {
+    font-size: var(--text-xs);
+    color: var(--color-text-muted);
+    line-height: 1.5;
     margin-bottom: var(--space-3);
+    padding-top: var(--space-2);
+    border-top: 1px solid var(--color-border);
   }
 
   &__remove-confirm-actions {