feat(tasks): All Tasks cross-project view + fix viewMode flash on project switch

[hudsonhrh]

Summary

Two related fixes on the tasks page, bundled into one PR because they touch the same code path:

1. Flash fix — viewMode (board/list/gantt) no longer resets to 'board' for a frame when you switch projects in List or Gantt view. The view state is now hoisted above the keyed TaskBoardProvider boundary instead of being reset on every project remount.

2. All Tasks view — new top-of-sidebar entry that aggregates every project's tasks into one cross-project view. Distinct accent treatment so it doesn't blend into the project list. Read-only for now (Add Task hidden, mutations toast "Open a project to make changes" — clicking a task navigates into the project's detail modal where everything works normally).

Screenshot

(See /tmp/poa-screenshots/all-tasks-final.png locally — header shows ""All Tasks 28 tasks across 3 projects"", sidebar shows the accent-tinted ""All Tasks · 28 tasks · 3 projects"" entry at the top, list view aggregates across projects with a project chip on each row.)

Flash fix

Root cause: MainLayout.js mounts <TaskBoardProvider key={selectedProject.id}> so each project gets a fresh TaskBoardContext (intentional — optimistic state must reset per project). TaskBoard lives inside that subtree and owned useViewMode. On every project switch, useViewMode's state reset to the 'board' default, then a useEffect read localStorage and switched to the user's stored mode → that's the visible flash.

Fix: views/useViewMode.js split into a TaskViewModeProvider (lives at the MainLayout level, sibling above the keyed boundary) and a thin useViewMode consumer. Same public API; viewMode now persists across project switches.

All Tasks view design

Sidebar entry — distinct but consistent:

Position	Top of sidebar, above the existing ""Projects"" heading
Visual	Rounded card with purple gradient accent, stacked-icon glyph, count subtitle (""N tasks · M projects"")
Selected state	Saturates the gradient + purple ring/shadow, matches the project-row selection pattern
Route	?projectId=__all__ (ALL_PROJECTS_ID sentinel — cannot collide with real on-chain project ids which are 0x-prefixed)

Data layer:

• AllTasksProvider (new) wraps the same TaskBoardContext that ListView / GanttView / TaskBoardDesktop already consume, so the existing views render unchanged
• aggregateAllTasksColumns(projects) merges same-status columns across every project and decorates each task with projectId + projectName for the chip
• All mutators (addTask, moveTask, editTask, deleteTask, applyForTask, approveApplication, assignTask, rejectTask) are no-op + toast; the All Tasks view is a browser, not an editor
• TaskColumn reads isAllTasks from context and hides the Add Task button accordingly
• TaskCard and TaskRow render a project chip when task.projectName is populated (only true in this aggregation)

Mobile project picker doesn't include All Tasks yet — desktop-only for v1.

Test plan

• yarn build clean
• Headless Chrome screenshot on Test6 confirms All Tasks entry + aggregation + project chips
• Navigate between projects in list/gantt view → no flash to board
• Switch to All Tasks → board, list, gantt all render aggregated tasks
• Add Task button is hidden in All Tasks board view
• Drag a card in All Tasks board → toast ""Open a project to move this task""
• Click a card in All Tasks → opens task modal in the right project context
• Switch from All Tasks back to a specific project → no regression on the per-project flow

🤖 Generated with Claude Code