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/sse-events.ts

@@ -5,7 +5,7 @@
  * and referenced by the frontend (`SSE_EVENTS` in `constants.js`).
  * Both files MUST be kept in sync.
  *
- * ~90 event constants organized by category:
+ * ~106 event constants organized by category:
  * - **Core** (1): init
  * - **Session lifecycle** (17): created, updated, deleted, terminal, idle, working, ...
  * - **Session: Ralph** (6): ralphLoopUpdate, todoUpdate, completionDetected, ...