fix(web-ui): make the kiro session picker load from metadata only

[ShotaroKataoka]

Summary

The Local-mode "Continue from a kiro session" picker parsed every .jsonl log under
~/.kiro/sessions/cli in full on each open, only to count prompts. On a real store
(655 sessions, 3.7 GB, largest log 354 MB) listSessions() took ~100 s; it now takes
~0.09 s with identical groups / recent results.

• Build the list from .json metadata only; drop messageCount from
  KiroSessionSummary, the picker row and the fork progress card (unused i18n keys
  sessionPicker.messages / forkProgress.sessionMeta removed from en + ja)
• Read a log only when the stored title is empty (60 of 655 locally), stopping at the
  first prompt — listing no longer scales with log size, and depends far less on
  kiro-cli's undocumented log format
• Picker dialog is now a height-capped flex column (DialogContent defaults to grid,
  so the list's flex-1 overflow-y-auto never applied and "Show all" was clipped)

Testing

• New unit tests: titled sessions never open the log; title fallback skips malformed /
  non-prompt lines and stops at the first prompt
• npx eslint src/ --quiet, npx tsc --noEmit, npm test (359 passed), npm run build:cloud
• en/ja key parity check, ash scan --mode local --fail-on-findings (passed)
• Manual: npm run dev:local — /api/agent/sessions 0.29 s warm, ?all=1 0.21 s;
  "Show all" list scrolls