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, 3 insertions, 3 deletions

diff --git a/frontend-vanilla/src/polling.test.ts b/frontend-vanilla/src/polling.test.ts
index fa4b62f..44ad7da 100644
--- a/frontend-vanilla/src/polling.test.ts
+++ b/frontend-vanilla/src/polling.test.ts
@@ -44,8 +44,8 @@ describe('Infinite Scroll Polling', () => {
             json: async () => []
         } as Response);
 
-        // Wait for interval (1000ms) + buffer
-        await new Promise(resolve => setTimeout(resolve, 1100));
+        // Wait for interval (3000ms) + buffer
+        await new Promise(resolve => setTimeout(resolve, 3100));
 
         // Check if apiFetch was called
         expect(apiFetch).toHaveBeenCalledWith(expect.stringContaining('/api/stream'));
@@ -58,7 +58,7 @@ describe('Infinite Scroll Polling', () => {
         Object.defineProperty(scrollRoot, 'clientHeight', { value: 200, configurable: true });
         Object.defineProperty(scrollRoot, 'scrollTop', { value: 100, configurable: true });
 
-        await new Promise(resolve => setTimeout(resolve, 1100));
+        await new Promise(resolve => setTimeout(resolve, 3100));
 
         expect(apiFetch).not.toHaveBeenCalled();
     });