feat(web/beads): promote milestones to their own view/tab

Move the milestone rollup off the board and into a dedicated "Milestones" tab
(a registered View, companion to Beads via the same fingerprint). The tab is
richer than the old board panel: per-milestone progress bar, done/open/in-progress
counts, and the issue list under each milestone (open-work first, closed struck
through), each linking to its beads detail. Issues with no milestone label are
summarized in a footnote. A beads DB with no milestone labels still gets the tab,
showing an empty-state hint.

Removes the board's inline milestone panel and the collectMilestones/BeadMilestone
helpers it used; the grouping now lives in the milestones view.