Snack shop owner - monthly break-even calculator - single-file web page, fixed costs in, the daily sales number out, and shrinkage on its own line showing the margin it eats

You are a coding agent building a tool for an
independent snack shop owner. Build a single-file
HTML tool (all inline, no external dependencies, no
network calls, opens by double-click): a monthly
break-even calculator.

The owner's problem, stated in the tool's
positioning: whether this month made money is often
felt two months late - rent rose, a hire came on, a
new competitor opened, and the books only say so
after the losses are old. The tool should let
someone look once each morning and know what today
has to sell to not lose.

Input section:
- Monthly fixed costs: rent / labor (including a
  salary line for the owner) / utilities and
  property fees / other fixed outlays, itemized
- Estimated margin rate: enterable directly, or
  derived from "monthly purchase cost / monthly
  sales" (both entry points)
- Shrinkage on its own line: an estimated combined
  rate for bulk-bin loss + damaged-pack write-offs
  + markdown discounts - it may never be folded
  into the margin field; it gets its own row
- Month-to-date sales + trading days elapsed

Calculations and output:
1. Break-even number cards: monthly break-even
   sales = fixed costs / (margin rate - shrinkage
   rate); the daily break-even figure. The margin
   used is "net of shrinkage" - print that phrase
   on the page; most break-even math skips
   shrinkage
2. Progress comparison: "should-have-reached-today"
   pro-rated daily vs actual cumulative, gap shown
   in color; the daily average the remaining days
   must carry (the catch-up line)
3. Sensitivity table: how much the break-even line
   rises per one point of margin lost, and per one
   point of shrinkage gained - side by side, so
   the owner sees shrinkage's leverage on
   break-even is no smaller than margin's
4. Method notes: every formula printed on the page,
   numbers updating as inputs change

Boundaries (printed on the page):
- No repricing suggestions, no sales forecasting -
  the tool does arithmetic; decisions are made by
  a person
- The owner's own salary line is required - an
  owner who doesn't pay themselves computes a
  falsely low break-even

Hard requirements:
- Data stays local (localStorage), exportable and
  resettable
- Preloaded sample data so results are visible on
  open
- Number cards print cleanly, sized for the inside
  of the counter

Why it is written this way

Shrinkage on its own line, never folded into margin, is the tool's most important cut. When an independent owner says "margin is around 35%," that is almost always the purchase-spread figure; the three-to-five points eaten by bin loss, damaged packs, and markdowns are habitually left out, and the break-even line runs systematically low - the books look profitable while the losses quietly age. Its own row plus the two-way sensitivity table (margin down a point beside shrinkage up a point) makes the invisible eater visible: its leverage on break-even matches margin's, and the actions that curb it (rotating bins, damage walks) are far easier than repricing.

The mandatory owner-salary line exists to puncture the owner-operator's signature illusion. An owner who doesn't count their own labor watches a break-even line get passed day after day while actually working for free. The tool doesn't humor that habit - it's written into the boundary clauses. Cut for the same reason: repricing suggestions and sales forecasts. The moment an arithmetic tool starts giving business advice, the owner starts outsourcing judgment to it, and this tool's whole value is making the numbers honest first.

The catch-up line turns the tool from monthly review into daily instrument. A break-even figure alone is static - glancing at "180k this month" on day 15 produces no action; "15 days left, 7,200 a day" translates the month's problem into today's. That is what the printable cards taped inside the counter are for: one glance while opening, one number for the day.

Variables