Fix theme performance regressions affecting mobile scroll

The new theme CSS files introduced several patterns that cause
scroll jank and memory pressure, especially on mobile:

- terminal.css: Full-viewport fixed pseudo-element with repeating
  gradient scanlines forced GPU compositing on every scroll frame.
  Now limited to desktop only with will-change layer promotion.
- codex.css/sakura.css: text-rendering: optimizeLegibility on body
  triggered expensive kerning/ligature computation on all text.
- codex.css: font-feature-settings forced text shaper on every glyph.
- codex.css: hyphens: auto required dictionary lookups during layout.
- style.css: transition: all on buttons and sidebar links caused
  unnecessary animation work during scroll hover state changes.
- main.ts: checkReadItems did O(n) individual querySelector calls
  per scroll tick; switched to single querySelectorAll batch query.
- Polling interval reduced from 1s to 3s (scroll handler already
  covers the normal case, polling is just a robustness fallback).

https://claude.ai/code/session_0187FXrbScDSWfbNEk9SfJaj

1 files changed, 2 insertions, 2 deletions

diff --git a/frontend-vanilla/src/style.css b/frontend-vanilla/src/style.css
index 0f7b909..fd51eff 100644
--- a/frontend-vanilla/src/style.css
+++ b/frontend-vanilla/src/style.css
@@ -163,7 +163,7 @@ html {
   padding: 0.3rem 0.8rem;
   margin: 0.1rem 0;
   border-radius: 8px;
-  transition: all 0.2s ease;
+  transition: background-color 0.2s ease, color 0.2s ease, opacity 0.2s ease, transform 0.2s ease;
   font-weight: 500;
   font-size: 0.85rem;
   /* Explicitly smaller sidebar links */
@@ -703,7 +703,7 @@ button,
   font-family: var(--font-heading);
   background-color: var(--bg-color);
   cursor: pointer;
-  transition: all 0.2s;
+  transition: background-color 0.2s, color 0.2s, border-color 0.2s;
   color: var(--text-color);
   text-decoration: none;
   display: inline-block;