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/regression.test.ts b/frontend-vanilla/src/regression.test.ts
index 972b221..7f72ed5 100644
--- a/frontend-vanilla/src/regression.test.ts
+++ b/frontend-vanilla/src/regression.test.ts
@@ -202,8 +202,8 @@ describe('Scroll-to-Read Regression Tests', () => {
         // Dispatch scroll on WINDOW, not mainContent
         window.dispatchEvent(new Event('scroll'));
 
-        // Wait for potential debounce/poll
-        await new Promise(resolve => setTimeout(resolve, 1100));
+        // Wait for potential debounce/poll (3000ms interval + buffer)
+        await new Promise(resolve => setTimeout(resolve, 3100));
 
         // Expect it to handle it
         expect(apiFetch).toHaveBeenCalledWith(expect.stringContaining('/api/item/12345'), expect.objectContaining({