feat(web): indicate session activity in date picker

[techotaku39]

Summary

Makes the session date-range picker show which dates contain session activity.

• keeps dates with matching session activity at the normal foreground color
• dims dates without session activity while leaving them selectable
• derives availability from the same session set used by the date filter
• preserves the existing selected endpoint and in-range highlighting

Follow-up to #1083.

Motivation

The date filter added in #1083 helps narrow a large session history, but every day currently has the same visual weight. Users still need to guess which dates contain sessions. Showing activity availability directly in the calendar makes useful dates visible at a glance without adding controls or preventing broader range selection.

Test plan

• UI coverage for normal styling on dates with session activity
• UI coverage for dimmed styling on dates without session activity
• UI coverage confirms a date without activity remains selectable
• Full web test suite: bun run test:web
• Monorepo TypeScript checks: bun typecheck
• Production web build: bun run build:web
• Manual verification on the port 3016 test deployment

AI disclosure

Implementation and tests were assisted by OpenAI Codex (GPT-5.6).

[github-actions]

Findings

• [Minor] Activity signal is color-only — the new hasSessionActivity state only changes text-[var(--app-fg)] vs text-[var(--app-hint)], while the day button label remains just the date. Screen-reader and forced-colors users get no session-activity indication, so the new feature is unavailable for them. Evidence web/src/components/SessionList.tsx:642
  Suggested fix:

  const dateLabel = date.toLocaleDateString()
  const buttonLabel = hasSessionActivity
      ? t('sessions.timeFilter.dayWithActivity', { date: dateLabel })
      : dateLabel
  
  <button aria-label={buttonLabel} title={buttonLabel}>

  Add matching sessions.timeFilter.dayWithActivity entries in web/src/lib/locales/en.ts and web/src/lib/locales/zh-CN.ts.

Summary

• Review mode: initial
• One minor accessibility gap found in the new date activity indicator. Residual risk: visual-only behavior around color themes was not exercised here.

Testing

• Not run (automation; review only, no PR code execution).

HAPI Bot

[github-actions]

Findings
No findings.

Questions
None.

Summary

• Review mode: follow-up after new commits
• No issues found in the latest full PR diff. The previous bot accessibility finding is addressed at the current head. Residual risk: review-only pass; tests/build were not rerun here.

Testing

• Not run (automation; review only).

HAPI Bot