Let me first read all the current components to understand the full system before making changes.

Now I have a complete picture of the codebase. The design document requests a major overhaul of the calendar component specifically: replacing the small colored dots with large circular stamps (70-90% of cell height) that overlay the date numbers, with an ink-stamp aesthetic (slight rotation, subtle texture). Let me rewrite the homework-calendar.tsx component to match these requirements.

Now let me also add some custom CSS for the stamp ink noise effect and update the globals with a keyframe for a subtle stamp press animation:

Now let me also update the page layout to give the calendar more vertical space by adjusting the split between calendar and day detail:

Now let me also update the app-header to match the title format requested ("2025년 3월 출석" style), and refine the stamp colors to be more vivid for stamps while keeping the overall pastel feel. Let me adjust the stamp color tokens to have bolder borders but softer backgrounds:

Now let me also update the app header to be cleaner and more aligned with the document's layout requirements (the month title is in the calendar itself now, so the header can be simpler):

Now let me update the page layout to give the calendar section more room and make the overall layout more balanced:

Let me also quickly update the stamp selector component's visual to match the new large-stamp aesthetic, making the selector buttons themselves more stamp-like:

The calendar has been completely redesigned to match the design document specifications. Here's what changed:

Homework Calendar - Replaced small colored dots with large circular stamps (82% of cell size) that overlay the date numbers. Each stamp has a bold colored border + lighter same-color background, with the date number centered inside in a matching bold color. A deterministic pseudo-random rotation (-4 to +4 degrees) per date gives each stamp a natural "hand-pressed ink" feel, along with a subtle SVG noise texture overlay at 4% opacity for an authentic stamp aesthetic.

Interaction - Stamped cells brighten slightly and gain a shadow on hover. Today's date always shows a ring highlight regardless of stamp status. Unstamped dates show plain gray numbers with no circle, creating clear visual contrast between attended and unattended days.

Supporting updates - The header was simplified, stamp color tokens were deepened for better contrast, and the stamp selector buttons now use bordered pill shapes matching the stamp aesthetic.

Let me read all the current files to understand the full system before implementing Version C.

Now I have a complete picture of the codebase. Let me implement all the Version C changes. The key modifications are:

1. Types: Add "black" to StampColor for "숙제 없었음"
2. App Context: Add selectedStamp state + toggle logic, update sample data with some "black" entries, add CLEAR_STAMP action for "숙제 없었음" override
3. Homework Calendar: Apply stamp via palette click (not auto), support "black" stamp rendering
4. Stamp Selector: Convert to palette-style toggle (click to select/deselect), add "숙제 없었음" button
5. Stats Panel: New achievement formula excluding "black" days
6. Day Detail: Support "black" stamp display
7. Slack routes: Skip notifications for "black" stamp days
8. Globals CSS: Add --stamp-black color tokens

Let me implement all changes:

Now update the globals.css to add --stamp-black color tokens:

Now update the app-context with the new selectedStamp state, toggle action, and updated sample data:

Now let's move to the calendar and stamp palette:

Update the calendar to support "black" stamps and use APPLY_STAMP_TO_DATE when a palette stamp is armed:

Now add "black" stamp rendering in the CalendarCell:

Now I need to also show a visual cursor indicator when a stamp is armed. Let me add a subtle ring to indicate that cells are "clickable to stamp":

Now pass isStampArmed to CalendarCell and update props:

Now add cursor styling for when stamp is armed:

Now rewrite the stamp selector to be a palette-style toggle:

Now update the DayDetail component to support the "black" stamp display and to remove the old per-date stamp selector props:

Now update the StampSelector usage in DayDetail -- it no longer takes props:

Now move to stats panel and Slack routes:

Now rewrite the stats panel with the new achievement formula:

Now update the Slack notification route to skip "black" stamp days:

Now update the cron route to also pass the stamp info:

Version C has been implemented across 8 files with all requested changes. Here is a summary of the diff-notable changes (all marked with [Version C] comments):

1. New "숙제 없었음" stamp (lib/types.ts): StampColor now includes "black", and AppState has a selectedStamp field for the palette.
2. Palette-style toggle (components/stamp-selector.tsx): Completely rewritten -- clicking a stamp arms it, clicking again disarms; 4 buttons (blue/yellow/red/black) with clear active/idle styling and a hint message.
3. Calendar cell click (components/homework-calendar.tsx): When a stamp is armed in teacher mode, clicking a date applies that stamp. Visual ring indicators show cells are clickable. Black stamp renders with gray-scale tones.
4. Achievement rate (components/stats-panel.tsx): New formula D_done / D_effective where black-stamp days are excluded from both numerator and denominator. A verification log (console.log) prints the breakdown on each calculation. Stats grid is now 4 columns including "없었음".
5. Slack skip (app/api/slack/notify/route.ts, app/api/cron/check-homework/route.ts): Both routes check for stamp === "black" and skip all notifications for those dates.