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, 1 deletions

diff --git a/frontend-vanilla/public/themes/sakura.css b/frontend-vanilla/public/themes/sakura.css
index f0fc990..48a1c0a 100644
--- a/frontend-vanilla/public/themes/sakura.css
+++ b/frontend-vanilla/public/themes/sakura.css
@@ -57,7 +57,8 @@ body {
   background-color: var(--bg-color);
   color: var(--text-color);
   -webkit-font-smoothing: antialiased;
-  text-rendering: optimizeLegibility;
+  /* text-rendering: optimizeLegibility removed -- causes expensive text
+     shaping on all content, leading to scroll jank on mobile. */
 }
 
 /* ---- Sidebar ---- */