feat: add DatePicker component

[patrickkuei]

Closes #13

Summary

• Adds a DatePicker component: a cyberpunk-styled single-date picker for booking forms, filters, and scheduling UI.
• API decisions: value/onValueChange (controlled) or defaultValue (uncontrolled) manage the selected Date; minDate/maxDate bound selectable days, rendering out-of-range cells dimmed and disabled rather than hiding them. The trigger is a read-only text <input> reusing Input's exact variant/size class logic (primary/secondary/danger/ghost) so it looks identical to other form inputs. The popover reuses DropdownMenu's click-outside/staged open-close mechanics (same timing, same transition classes) rather than reinventing overlay plumbing, and matches Modal/Select's neon-bordered surface treatment. The calendar grid follows the WAI-ARIA date-picker pattern (role="grid"/"row"/"columnheader"/"gridcell") with a roving-tabindex focused cell — arrow keys move a day/week at a time and can cross month boundaries, Home/End jump to the edges of the focused week, PageUp/PageDown step a month, and the view always renders a fixed 6-week grid so row count never jumps between months.
• Deferred scope: per the issue's own suggestion ("single-date selection to start, range mode can be a follow-up"), this ships single-date selection only. Range (two-date) selection is a natural follow-up once this is proven out — flagging it explicitly rather than blocking this PR on it.

Changes

• Added DatePicker (src/components/DatePicker.tsx) — value/defaultValue/onValueChange, minDate/maxDate, label, placeholder, variant, responsive size, disabled, helperText, error, formatDate, ariaLabel, className, id. No new runtime dependencies — all date math (month/week arithmetic, grid building) is hand-rolled.
• Exported DatePicker/DatePickerProps from src/components/index.ts.
• Stories (DatePicker.stories.tsx): Default, Primary/Secondary/Ghost variants, WithBoundedRange (14-day window demo), sizes (Small/Medium/Large), WithError, Disabled, Controlled, and an AllVariants render story (cyberpunk copy throughout, e.g. "Neural Sync Window", "Extraction Window", "Cold Storage Timestamp").
• Tests (DatePicker.test.tsx, 19 cases): renders trigger without crashing, aria-haspopup/aria-expanded/aria-controls wiring, opens on click, formatted value display, day selection (uncontrolled + controlled, without self-managed state), minDate/maxDate disabling out-of-range days, aria-current="date" on today, ArrowDown opening the calendar, Arrow key grid navigation (day and week steps), PageUp/PageDown month stepping, Enter selection, Escape close + focus return, click-outside close, disabled blocks opening, custom formatDate, error state + aria-invalid, responsive size.
• Wired DatePicker into scripts/generate-manifest.js (CATEGORY_MAP → Forms, alongside Select/RadioGroup; DOC_SUMMARIES) and regenerated docs via npm run docs:generate — diff reviewed, only DatePicker-related lines changed.
• Added a CHANGELOG entry under [Unreleased].

Test plan

• npm run lint / type-check / test -- --project=unit all pass (314 tests, including 19 new for DatePicker)
• Storybook: Default + variants + sizes + disabled + AllVariants render correctly
• Keyboard nav + screen reader smoke test

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