Tags should show their original tinted colour, and Settings should offer the colour they actually display

[ericgriffin]

What we want

Tags should look the way they looked before #2255: a pale tinted chip carrying
the diver's colour, not a chip flooded with that colour at full strength. The
colour swatches in Settings > Manage > Tags should then show that display
colour, rather than the raw saturated hex the tag is stored as.

#2255 fixed the real complaint in #2254, that a tag's colour in Settings did
not match the colour the tag showed anywhere else, by moving the chips to the
stored colour. The mismatch is better resolved the other way round: keep the
quiet chips and correct what Settings promises.

Why the tint has to become opaque

A 15% translucent fill is not a colour, it is a recipe whose result depends on
the surface behind it. That is what #2254 measured: the amber tag #F59E0B
resolved to F7E7CF on a detail card, F2E8D7 on an ordinary dive row and
C4C2B7 on a selected one, where the row's blue-grey bled through.

So a swatch cannot represent "the actual display colour" while the chip stays
translucent, because there is no single such colour. The tint has to be
computed once against a fixed reference surface and then painted opaque. The
pixel on a plain card is unchanged from the pre-#2255 chip; what goes away is
the drift onto selected rows, and the swatch becomes able to tell the truth.

This is the treatment lib/features/courses/presentation/course_status_colors.dart
already uses for the course status card.

Scope

• TagChip stays the one widget behind every tag, and repaints as the tinted
  recipe: blended opaque fill, full-hue border, hue-preserving label.
• One tint, not two. The old code used 0.15 in dive lists and 0.2 on
  cards, which would give a tag two display colours again.
• The label keeps the tag's hue but is darkened until it clears WCAG AA
  against the fill. The pre-fix(tags): show a tag in its own colour, not a tint of it #2255 chips wrote the label in the raw
  tag.color, which on its own 15% tint is about 1.7:1 for the pale hues.
• TagColorPicker swatches become chip-shaped, drawn by the same colour
  function, in all three places it appears: both dialogs in TagManagePage
  and TagMergeSheet.
• The Manage list row's solid CircleAvatar becomes the tag's real chip, so
  the spot Tag colour in the app does not match the colour set in Settings > Manage > Tags #2254 measured agrees with the rest of the app.
• The 48/32 dp close-button tap target added in fix(tags): show a tag in its own colour, not a tint of it #2255 stays. It is an
  accessibility floor and has nothing to do with colour.