feat(kanban): badges done/locked + drag locked en mismo column + migrations + UX stickers

- badges: locked → tiempo bloqueado; done → fecha completion + total lead time; otherwise → tiempo en columna
- locked cards: drag permitido dentro de mismo column (cross-column rejected con notification)
- card field: locked_at desde JOIN card_lock_history (open period)
- migrations: refactor a embed.FS, archivos 002-005 extraidos de ensureColumns; ensureColumns queda como backstop
- stickers UX: opacidad 1, debajo del texto, picker estable (useRef), boton entra directo a modo con 😀, popover cierra outside, cards done filter brightness
- format: formatDateTimeShort

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

migrations/002_add_stickers.sql

@@ -0,0 +1,4 @@
+-- Add stickers column to cards. Idempotent ALTER pattern in db.go ensureColumns.
+-- Stickers persist as JSON array: [{"emoji":"🔥","x":0.5,"y":0.5}, ...]
+-- x, y in [0, 1] relative to card dimensions for resize survival.
+ALTER TABLE cards ADD COLUMN stickers TEXT NOT NULL DEFAULT '[]';