feat(colors): share the custom HEX history across every picker

[kirillDevPro]

Closes #488.

What & why

A colour typed into a picker's HEX field was remembered only by the Badges settings tab, which
kept its own reuse list inside the portable badges.json. Every other picker — core colour,
strategy colour fields, order lines, chart figures, labels, the arbitrage editor, News tags —
made the user retype the same shade from its code each time. That is what the tester was actually
asking for: the palette is already identical everywhere, but only badges remembers.

The history now lives in its own cfg/custom_colors.json, owned by moon-core, and every picker
seeds from and writes to one app-global entity. Most-recent-first, de-duped, capped at 20 to match
MoonUI's own MAX_CUSTOM_COLORS, above which a seeded list is silently trimmed anyway.

Notes for review:

• Not an AppConfig field, on purpose. Remembering a colour is immediate app-local state, so
  Cancel in Settings cannot roll it back and Save is not required to keep it. That is also why it
  is a standalone file rather than a key in settings.toml, which goes through the draft/preview
  machinery.
• badges.json keeps its list and is imported once, the first time the new file is absent, so
  a downgrade still finds it. BadgesConfig::custom_colors therefore survives as migration-only
  data; parse_share still preserves it across a paste, for the same reason it always did.
• A corrupt badges.json imports as empty rather than failing — the new file is never created
  on the error path, so a hard error would have left the history unpersisted on every later launch
  too. BadgesConfig::load already tolerates that same corruption.
• The fan-out never rebuilds editor state. The badges tab's old sibling-fan-out existed
  precisely to avoid tearing down every row's MoonInputState, including the hex field being
  edited; the shared subscription only pushes swatches.
• The second commit bumps MoonUI to pick up fix(color_picker): reveal scrollable palettes MoonUI#65, the other half of the issue:
  the swatch grid now shows that it scrolls.

How to verify

cargo test -p moon-core — 1919 passing. cargo fmt --all -- --check and
cargo check -p moon-core -p moon-ui-gpui --locked clean.

Checked in a live debug build against real cores: typing a HEX in the Connections tab's core-colour
picker writes cfg/custom_colors.json, and the same colour then leads the grid in the Lines tab's
order-line picker, which has its own unrelated value. The scrollbar is visible on open without
interaction.

Not verified: migration of a non-empty legacy list (the test profile's was empty) and the
corrupt-config path — both are named in the branch notes as unproven, with the mutations that would
catch a regression.

https://claude.ai/code/session_017mwXJTSXLJdEMfjXSvWpS4