feat(web): vertical minimap for user messages

[tyulyukov]

Summary

Ports t3code#2348 by @akarabach.

Adds a compact minimap rail on the right side of the chat showing one dash per user message in the current thread. Hover expands it into a preview menu; clicking a row scrolls the conversation to that message. The active dash tracks scroll position. Hidable via a new "Chat minimap" toggle in the General settings panel.

What changed

File	Purpose
apps/web/src/components/chat/ChatMinimap.tsx (new)	The component itself: collapsed dashes strip, hover-expanded preview menu, click-to-navigate, active-dash tracking.
apps/web/src/components/chat/ChatMinimap.logic.ts (new)	Pure helpers: `selectUserMessageMinimapEntries`, `selectVisibleMinimapEntries`, `computeActiveMinimapIndex`.
apps/web/src/components/chat/ChatMinimap.browser.tsx (new)	Browser tests covering empty state, dash count, scroll-driven activation, thread reset, hover-to-open, click-to-navigate.
apps/web/src/components/ui/preview-card.tsx (new)	Thin wrapper around `@base-ui/react/preview-card` for hover `delay` / `closeDelay`.
apps/web/src/components/chat/MessagesTimeline.tsx	Wraps the LegendList in a positioned container, adds minimap, conditional right-padding when minimap is visible.
apps/web/src/components/chat/MessagesTimeline.logic.test.ts	New tests for `selectUserMessageMinimapEntries` and `computeActiveMinimapIndex`.
apps/web/src/components/settings/SettingsPanels.tsx	Adds the "Chat minimap" toggle and dirty-state tracking.
packages/contracts/src/settings.ts	Adds `hideChatMinimap` (default `false`) to `ClientSettingsSchema` + `ClientSettingsPatch`.
Persistence tests	Switched to `Schema.decodeSync(ClientSettingsSchema)({…})` for default-tolerant fixtures.

MarCode adjustments vs upstream

• Imports changed from `@t3tools/contracts` to `@marcode/contracts`.
• `ClientSettingsPatch` additions limited to `hideChatMinimap`. Upstream PR also brought along `sidebarProjectGroupingMode` / `sidebarProjectGroupingOverrides` because they were in upstream main when the PR was based; MarCode already has those in `ClientSettingsSchema`, so no patch entry needed for this PR.
• `SettingsPanels.useSettingsRestore` restore-list adds only "Chat minimap" (upstream brought "Task sidebar" too; not yet in MarCode).
• `MessagesTimeline` keeps MarCode's existing `rows = useMemo(...)` (upstream introduced a `useStableRows` wrapper for structural sharing; MarCode doesn't have it and the minimap doesn't require it).
• LegendList wrapped in a `@container/chat relative h-full` div with a conditional `pr-2` when the minimap is visible (the parent in `ChatView` already provides `px-3 sm:px-5`, so right-padding here is incremental).
• `ChatMinimap` `threadKey` wired to `threadId` (upstream uses `routeThreadKey`, a prop MarCode doesn't have).
• Test fixtures in `clientPersistence.test.ts` and `localApi.test.ts` refactored to `Schema.decodeSync(ClientSettingsSchema)({…non-default})`, matching upstream's cleaner pattern while preserving MarCode's intent of round-tripping non-default values.

Test plan

• `bun --cwd packages/contracts run typecheck` — clean
• `bun --cwd apps/web run typecheck` — clean
• `bun --cwd apps/desktop run typecheck` — clean
• `bun --cwd apps/web run test` — 1139 passing (+35 new for the minimap)
• `bun --cwd apps/desktop run test` — 97 passing
• Manual smoke (reviewer):
  • Open a thread with 10+ user messages → dashes appear, last visible dash is active
  • Scroll up/down → active dash updates in sync
  • Hover the rail → menu expands; mouse-leave → menu collapses after a short delay
  • Click any preview → conversation scrolls to that message and menu closes
  • Switch threads → active highlight resets, menu closes if open
  • Toggle "Chat minimap" off in Settings → rail disappears, padding collapses

Upstream

Original PR: pingdotgg#2348