feat(ux): PR 1.3 — replace 0→100 hover-reveals with calm baseline

Affordances on MicroSteps rows (thumbs/edit/speaker/just-start) and
transcript-viewer segments (star, copy, delete) used opacity-0 →
opacity-100 on group-hover, which (a) hid them from keyboard users
until focus and (b) violated the "always-visible affordance" line in
the Kon design audit.

Replace with opacity-30 baseline + group-hover:opacity-100 +
focus-visible:opacity-100 (MicroSteps) / focus-within:opacity-100
(viewer, since the actions sit inside an interactive segment row).
Result: affordances are calm but visible at rest, lift on hover or
keyboard focus, never invisible.

HistoryPage was checked — its row checkbox at line 757 is already
opacity-50 → opacity-100 on hover, the pattern this PR is moving
toward, so no change needed there.

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

src/routes/viewer/+page.svelte

@@ -534,7 +534,7 @@
             >
               <!-- Star -->
               <button
-                class="mt-0.5 flex-shrink-0 {seg.starred ? 'text-accent' : 'text-text-tertiary opacity-0 group-hover:opacity-100'}"
+                class="mt-0.5 flex-shrink-0 {seg.starred ? 'text-accent' : 'text-text-tertiary opacity-30 group-hover:opacity-100 focus-within:opacity-100'}"
                 onclick={(e) => { e.stopPropagation(); toggleStar(seg._idx); }}
                 aria-label={seg.starred ? "Unstar" : "Star"}
               >
@@ -570,7 +570,7 @@
                 </p>
               {/if}
               <!-- Actions (hover reveal) -->
-              <div class="flex items-center gap-1 opacity-0 group-hover:opacity-100 flex-shrink-0">
+              <div class="flex items-center gap-1 opacity-30 group-hover:opacity-100 focus-within:opacity-100 flex-shrink-0">
                 <button
                   class="text-text-tertiary hover:text-accent"
                   onclick={(e) => { e.stopPropagation(); copySegment(seg._idx); }}