serve: new-task modal UI

[andreiRS]

Part of the serve-board spec (docs/specs/serve-board.md), architecture in docs/adr/0016-board-server-reuses-core-in-process.md.

Effort

M — modal UI + create flow.

What to build

A "New task" modal: title (required) + body (markdown) + effort picker. Submits to POST /tasks; the new card appears in backlog (via the optimistic/SSE path). No lane or attendance picker in v1 (lands attended, in backlog).

Acceptance criteria

• Creating a task from the modal adds it to the backlog lane and persists (one commit).
• Title is required; empty title is prevented/handled with feedback.
• The created card reflects the chosen effort.

Blocked by

#18, #15

[andreiRS]

Done on feat/serve-board. Green: 6bcde75 (new-task modal — optimistic create + reconcile). Review fix-up: dd9dcbf (non-deprecated SyntheticEvent<HTMLFormElement> in place of deprecated React.FormEvent; in-flight submittingRef guard against rapid double-Enter double-create).

Gates: tsc --noEmit clean (root + web), web build clean.

Browser-verified end-to-end on the single-origin production server (not the dev proxy):

• Modal renders: required title, raw-markdown body textarea, low/medium/high effort picker (medium default); create disabled while title empty.
• Happy path: create lands in backlog with the chosen effort + persisted body, real server id (Review remediation: teach the dependency graph about archive #8, not the -1 placeholder), persists as exactly ONE commit (task: new #8 …), survives reload.
• Failure path: against a dirtied store the create surfaces a STORE_DIRTY toast and removes the optimistic placeholder (no stuck/duplicate card).
• Double-submit guard: rapid triple-Enter in the title field produced exactly one task / one commit (Review remediation: floor Short ID allocation + doctor detection #9).