Rework the visa-free calculator

[ialakey]

The visa run calculator assumed every user gets 30 days from a single entry
date, and it counted the remaining days wrongly.

The counting bug

Remaining days were computed by subtracting two DateTimes and reading
.inDays, which truncates. Entering on 1 January with a 30-day allowance:

On	It said	Correct
28 Jan 10:00	1	3
29 Jan 10:00	0	2
30 Jan 10:00	0	1

The last three days of a stay all read as "0 days left" — the days when the
number matters most.

Not everyone is on the same rule

The guide's own calculator article sets out three regimes, and the screen now
asks which one applies:

Rule	Who
30 days per entry	Russia, Belarus, China, Kazakhstan
90 days in any 180	EU, USA, Ukraine and most others
30 days in any 365	Bahamas, Barbados, Colombia, Indonesia, Jamaica, Paraguay, St Vincent

This matters because a visa run only resets the counter under the first rule.
Under a rolling window, leaving and coming back buys nothing — and the screen
now says so instead of implying otherwise.

A rolling window also cannot be answered from one date: it counts every day
spent in the country across the window, however many trips that took. So the
screen keeps a list of trips. The single date the old screen stored is
carried over on first run, so nobody loses what they had entered.

The arithmetic is its own file, and tested

lib/data/visa_rule.dart holds the calculation with no Flutter in it, under
15 unit tests. Two things it gets right that are easy to get wrong:

• Both endpoints count. Enter 1 January, leave 2 January — that is two
  days of stay, as the guide states.
• Calendar days are anchored to UTC midnight. Duration(days: 1) is
  exactly 24 hours, so adding days to a local date across the last Sunday in
  March gains an hour and can drop a whole day from a difference. A test
  covers the boundary.

The screen

A day counter with a progress ring, the must-leave-by date, and an overstay
warning when the allowance is gone. The deadline can be turned into a real
reminder — that reuses the existing deadlines feature rather than adding a
second list of dates to the app — or exported to the calendar as an all-day
event.

The old screen's animated GIF icons rendered as flat black shapes, its empty
state claimed "29 days left" next to a blank date, and it formatted every
date with a hardcoded Russian y г. suffix regardless of language. All gone.
Six localization keys the old dialog owned are removed; the new strings are
in both languages.

One layout fix worth knowing about

Buttons placed directly in a Row now sit in Expanded. The app's button
theme asks for a full-width minimum size, and a Row hands its children an
unbounded width — together they assert, and the bottom sheet was rendering
with no height at all: visible as a dimmed screen with nothing on it. It is
noted in the theme next to the minimum size that causes it.

Verification

• dart format, flutter analyze (0 issues), 59 tests — 15 for the rules,
  6 driving the screen itself, including one that asserts the sheet has a
  non-zero size so the invisible-sheet bug cannot come back
• Exercised on an Android 16 emulator in both themes: adding a trip, switching
  rules and watching the count change, creating the reminder, and the trips
  list