fix: restore mobile terminal scrollback — use JS scrollLines() instead of broken native scroll

xterm.js DOM renderer doesn't populate .xterm-viewport's scroll area (the div
is empty, scrollHeight === clientHeight), so native CSS scrolling via
touch-action:pan-y and overflow-y:scroll had nothing to scroll. Desktop worked
only because the wheel handler called terminal.scrollLines() directly.

- Replace split mobile/desktop touch handlers with unified JS-driven handler
  that converts touch deltas to terminal.scrollLines() calls (with pixel
  accumulation for slow swipes and momentum scrolling)
- Change touch-action from pan-y to none on terminal elements so browser
  doesn't fight the JS handler
- Remove now-unnecessary xterm-viewport position/overflow/z-index overrides
  and iOS -webkit-overflow-scrolling rules
- Fix _shrinkPaddingToFit() arithmetic (was adding gap instead of subtracting)
- Minor: add route-helpers.ts to CLAUDE.md, fix sse-events.ts comment count

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/web/public/mobile-handlers.js

@@ -446,7 +446,7 @@ const KeyboardHandler = {
       const gap = container.clientHeight - app.terminal.rows * cellH;
       if (gap > 0 && gap < cellH) {
         const currentPadding = parseInt(main.style.paddingBottom) || 0;
-        main.style.paddingBottom = currentPadding + gap + 'px';
+        main.style.paddingBottom = Math.max(0, currentPadding - gap) + 'px';
         if (app.fitAddon)
           try {
             app.fitAddon.fit();