Tasks: remember the List/Board choice per project

[builtui]

Fixes #1702

Problem

The Tasks view mode lives only in the URL, and every route into a project pinned the list:

Route in	Before
Sidebar project click	current route's view, else "list"
Task breadcrumb	hardcoded view: "list"
New project dialog	hardcoded view: "list"
Bare <projectId> subPath (reload, deep link)	parsed to "list"

So picking Board, opening a task, and coming back landed on the list — every time. The two neighbouring preferences already persist: sidebar collapsed state (shell/sidebar-preference.ts) and list filters/sort (views/list/list-preference.ts).

Change

A project route's view is now TaskViewMode | null, so "no view named" is distinct from "list". The shell resolves a null view through a new shell/view-preference.ts — the same client-local localStorage boundary the sidebar and list preferences use, so one client does not rewrite another client connected to the same bb server.

• The topbar toggle is the only place a user picks a view, so navigating with an explicit view is what records the choice (done once in the shell's navigation wrapper).
• The three routes that hardcoded "list" now name no view and let the preference answer.
• A project opened for the first time follows the view most recently chosen anywhere, then the list — switching projects keeps the feel of the old carry-over behaviour instead of snapping back.
• Storage handling mirrors list-preference.ts: unknown values are ignored, corrupt documents read as unset, and a document written by a newer client is left untouched rather than down-converted.

An unknown marker (?view=kanban) now resolves through the preference instead of silently meaning "list".

Tests

• shell/view-preference.test.ts — fallback chain, per-project isolation, corrupt documents, future-version write refusal.
• shell/shell.test.tsx — round-trips the marker-less project route; toggling Board then reopening the bare project URL renders the board; per-project choices stay apart while an unseen project follows the last one used; the sidebar navigates without pinning a view; toggling still works when storage rejects writes.

turbo run typecheck test --filter=bb-plugin-tasks: 36 test files pass, typecheck clean.

Notes

Client-local UI state only — no contract, RPC, CLI, or docs surface changes.

AGENT GENERATED: by Claude Opus 5