feat(web): onboarding polish - welcome shortcuts, Hardcore Mode, all-day nudge convert

[tyler-dane]

Summary

• Welcome dialog: u/i/s shortcuts for Sign up / Log in / Start Now, with keycap hints on each button
• Renamed "keyboard-only mode" copy to "Hardcore Mode" across the sidebar indicator, shortcut legend, command palette, and tour; the sidebar (and event-jump mode) now advertises only Esc as the exit gesture (Shift Shift still works, just isn't advertised)
• Shortcut tip text (e.g. "Hold Shift and press an arrow...") now renders key names as styled keycaps instead of plain words
• Shift+ArrowDown on a focused all-day event converts it to a 60-minute timed event, landing at the visible grid scroll position (mirrors the existing mouse drag-to-timed conversion); edge-focused all-day events are unaffected
• Refreshed welcome modal heading/body copy and the demo-events banner copy

Test plan

• bun run type-check passes
• bun run lint passes (0 errors; pre-existing unrelated warnings only)
• bun test:web passes (one pre-existing unrelated flake in DayCalendarGrid.test.tsx, reproduced identically on main)
• Manually verified in a running dev build: welcome modal keycaps + u/i/s shortcuts, demo banner copy, sidebar tip keycap rendering, and the all-day -> timed Shift+ArrowDown conversion in Week view

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