feature(web/monitor): start session modal with pre-flight + 5-min confirmation

[jamby77]

Summary

PR 9 of 25 in docs/plans/specs/monitor-command/plan-implementation.md. Stacked on top of #171 (PR 8). Lands the start-session modal with the full pre-flight context the spec describes: provider restrictions, ACL state with the exact ACL SETUSER snippet when +monitor is missing, the health-gate verdict, and a throughput / capture-size estimate.

• New components (one-per-file convention):
  • pages/monitor/start-session-modal.tsx — form + lifecycle
  • pages/monitor/preflight-panel.tsx — 4-section read-only display
• API client gains preflight() and startSession()
• Monitor.tsx grows a Start session button in the header that opens the modal; on success, invalidates the sessions list query so the new row shows immediately rather than waiting for the next 5 s poll
• Form fields:
  • Duration: integer + unit selector (seconds / minutes); minimum 1
  • Requested by: optional free-text, sent to the API and shown in the sessions table
• 5-minute confirmation guard: if the chosen duration exceeds 5 min, the first Submit click swaps the panel to a yellow warning ("Sessions over 5 minutes can produce significant load. Confirm to proceed.") and changes the primary button to "Yes, start session". Second click fires the start.
• Pre-flight refreshes whenever duration changes (so the size estimate stays accurate as the user adjusts), via useEffect with cancelled-flag cleanup.
• State reset on close: duration / unit / requestedBy / preflight / confirming / error all clear when the modal closes, so reopening always shows defaults. Confirmation auto-clears when duration drops back below 5 min.

Test plan

• pnpm --filter web exec tsc --noEmit → exit 0
• No new lint errors in touched files
• Run dev: MONITOR_DEV_PREVIEW=true pnpm dev:api and VITE_MONITOR_DEV_PREVIEW=true pnpm dev:web
• Browser: click Start session → modal opens, pre-flight panel populates within ~1 s
• All four pre-flight sections render: Provider (with restrictions list when applicable), ACL (badge + raw rules + remediation snippet when needed), Health gate (allow/skip badge + signals table), Throughput estimate (ops/sec, output KB/s, estimated lines + bytes)
• Duration to 6 minutes → first Submit shows the amber confirmation panel with the exact spec wording; primary button becomes "Yes, start session"
• Drop duration back to 30 s → confirmation panel clears automatically
• Cancel → reopen → form is back to defaults (30 s, no requestedBy)
• Submit a 3 s session with requestedBy:test → modal closes; row appears in the table within the 5 s poll window with that requestedBy value; status transitions running → completed
• Drop the API dev preview gate → modal Submit returns 404; error appears in the modal red text

Notes for reviewers

• Bug caught and fixed during live testing. The original useEffect(open) only cleared preflight / confirming / error on close; duration / unit / requestedBy carried over. A user who closed the modal at 6 min would still see 6 min when they reopened — and the unrelated state actually tripped the confirmation guard on what looked like a fresh open. Now everything resets on close.
• Net diff: 425 lines. Slightly above the 400 target. Composition: modal 196 + preflight panel 134 + API client extensions 72 + Monitor.tsx wiring 24. Splitting modal vs panel would have left an orphan PR.
• Web-side unit tests still deferred. The pure helpers (formatBytes, formatDuration, the percent / threshold formatters) are cheap to cover and should land alongside the live-tail UI in PR 10 as a small batch — flagged.
• Screenshots saved to docs/assets/:
  • pr9-start-session-modal.png (initial pre-flight)
  • pr9-confirmation-dialog.png (5-min guard)

Stacked PR

Base branch is feature/monitor-web-sessions-list (#171), so the diff shown is ONLY PR 9 changes.