feat: meeting coordination UI — Schedule meeting from connections list

[jlunder00]

Summary

• Adds stores/meetings.ts — Pinia store with requestMeeting, fetchMeetings, cancelMeeting; computed openMeetings and pendingRequestIds; full TDD (13 tests)
• Adds types/meetings.ts — TypeScript mirrors of backend request/response shapes
• Adds ScheduleMeetingModal.vue — per-connection modal with duration selector (15/30/60/90 min), pre-populated slot checklist (next 5 weekdays × 9am/1pm/4pm, user-toggleable), optional context note; Escape/backdrop-click/auto-focus keyboard support
• Extends ConnectionsSection.vue — "Schedule meeting" button per accepted connection, "meeting pending" badge when an open request exists, fetches open meetings on mount and refreshes after submission

Bug discovered and fixed during review

list_connections_for_user returned only UUIDs for connected users — but POST /api/meetings/request resolves targets via get_user_by_username. Without this fix, every "Send request" click would 404. Fix: SQL now JOINs the users table to expose other_username. Connection TypeScript type gains other_username: string. Connection rows now show the human username rather than a truncated UUID.

Test plan

• Settings → Connections: accepted connections show "Schedule meeting" button
• Click "Schedule meeting" — modal opens with correct username in title
• Pre-populated slot checkboxes span next 5 weekdays × 3 daily slots; unchecking reduces count
• Duration buttons highlight selection; note textarea accepts freeform text
• Submit with ≥1 slot selected → POST succeeds → modal closes → "meeting pending" badge appears on the connection row
• Submit with 0 slots → warning message shown, send button remains disabled
• Escape / backdrop click closes modal without submitting
• API failure shows inline error inside the modal

Checklist

• All 545 tests pass
• tsc --noEmit exits 0 (zero type errors)
• vite build exits 0
• pr-review-toolkit:code-reviewer ran — all findings addressed
• ACCOUNTING.md updated