feat(tui): add help overlay panel and simplify footer hints

[TSchonleber]

Summary

Adds an in-app help overlay toggled by ? and simplifies the footer to show only primary keys.

Changes:

• components.tsx: New HelpPanel component listing all keyboard shortcuts (?, q, r, p, tab, up/down, enter, w). Exported HELP_ENTRIES array for testability. Footer condensed to [q] quit · [r] refresh · [p] pause · [?] help.
• dashboard.tsx: Added showHelp state toggled by ? key. When active, the help panel replaces the main panes; header and footer remain visible. Works in non-raw terminals (plain text, no raw-mode-only features).
• tui.test.ts: New test verifying HelpPanel renders all entries. Existing footer test updated for new format.

Review & Testing Checklist for Human

• Run pnpm strand tui in a real terminal and press ? — help panel should appear with all 8 shortcuts listed, press ? again to dismiss
• Verify footer now shows only [q] quit · [r] refresh · [p] pause · [?] help plus the refresh timestamp
• Confirm other keys (q, r, p, tab, arrows, enter, w) still work as before when help is closed

Notes

• Scoped edits to dashboard.tsx, components.tsx, and tui.test.ts only — no other files touched.
• The pre-existing biome warning in src/auth/env-store.ts is unrelated.

Link to Devin session: https://app.devin.ai/sessions/17b0f31d8cd94b84b8ae405ff0c89fac
Requested by: @TSchonleber